Bukhari

Hadith Commentary:
In the narration of Muslim, there is an additional detail: and selling salt for salt is usury (riba) except when it is hand to hand (immediate exchange).
In any case, when any of these items are sold in exchange for their own kind, it is necessary that both are equal in weight or measure, and that the transaction is conducted on the spot (immediate payment).

Hadith Commentary:
(1)
There is brevity in this narration.
The detailed narration is as follows:
“Gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, and salt for salt—these items must be exchanged in equal amounts and hand to hand (immediate exchange).
Whoever gives more or takes more has engaged in usury (riba).
Both the one who takes usury and the one who gives it are equal in sin.”
(Sahih Muslim, Al-Musaqat, Hadith: 4061(1587))

(2)
According to the majority of jurists, there are two types of usury (riba):
➊ Riba al-Fadl:
Selling two items of the same kind with an increase or decrease (i.e., unequal amounts).
➋ Riba al-Nasi’ah:
There is no increase or decrease, but the transaction is such that one side pays immediately and the other side pays later (i.e., deferred payment).

The above hadith discusses the mutual exchange of food commodities, stating that the exchange of items of the same kind is permissible only when it is equal in amount and hand to hand (immediate exchange).
If one kind is exchanged for another kind, then an increase or decrease is permitted, provided the transaction is hand to hand (immediate exchange).
(Sahih Muslim, Al-Musaqat, Hadith: 4063(1587))

The ahadith mention the exchange of only six items:
Gold, silver, wheat, barley, dates, and salt.
The Zahiri scholars restrict this ruling to these six items, but all other schools of thought make an analogy (qiyas) to include other items as well.
In our view, this latter position is correct, because just as wheat is the staple food commodity in Pakistan and its neighboring countries, similarly, in the Far East (Malaysia, Indonesia, Japan, Korea, etc.), rice is the staple food.
In the Arab lands and surrounding regions, dates hold the same status as apricots do in the northern areas of Pakistan such as Baltistan, and as raisins do in the Mediterranean regions. Therefore, these items should be analogized to wheat, barley, and dates.
In any case, there are conditions for exchanging food items of the same kind:
They must be equal in amount and exchanged hand to hand (immediate exchange).
And if different kinds of commodities are being exchanged, then the only condition is that the transaction must be hand to hand (immediate exchange); an increase or decrease is permissible in such cases.

Hadith Footnote:

The linguistic analysis of the word "haa’ wa haa’" is explained by Allamah Shawkani rahimahullah as follows:

(إلَّا هَاءَ وَهَاءَ)
With elongation (madd) in both words and with the opening (fath) of the hamzah. It is also said:
With kasrah, and it is also said:
With sukoon, and the shortening without hamzah has been reported, but this was considered incorrect by Al-Khattabi, and An-Nawawi refuted him and said:
It is correct, but rare. The meaning is "take" (khudh) and "give" (haat). It has also been reported with the addition of a kasrah on the kaf, and it is said:
"Ha’i" with kasrah of the hamzah means "give" (haat), and with fath it means "take" (khudh). Ibn Al-Athir said:
"Haa’ wa haa’" is when each of the two parties in a sale says "haa’" and gives what is in his hand. It is also said:
Their meaning is "take" and "give". He said:
Other than Al-Khattabi, some allow sukoon in it. Ibn Malik said:
"Haa’" is an action noun (ism fi‘l) meaning "take". Al-Khalil said:
"Haa’" is a word used at the time of handing over. The intent of the statement:
"Haa’ wa haa’" is that each of the two contracting parties says to his companion:
"Haa’", so they both take possession in the gathering (majlis). (Nayl)

The summary of the meaning is that the word "haa’" is used with elongation (madd) and with both fath and kasrah of the hamzah. Some have also said it is used with sukoon.
Its meaning is "khudh" (take)
and "haat" (i.e., give).
It is also said that "haa’" with kasrah of the hamzah means "give" (haat), and with fath it means "take" (khudh).
Ibn Al-Athir said that "haa’ wa haa’" is that both the buyer and seller give to each other.
The buyer gives the money and the trader gives the goods, so its translation is "hand to hand", as if both take possession in the same gathering.

Hadith Commentary:
(1)
The reasoning of Umar radi Allahu anhu can only be fully established if the wording of the hadith is as follows:
"To sell gold for silver is usury except when it is hand to hand." Accordingly, in another narration, these exact words are reported, which Imam Bukhari rahimahullah has also mentioned.
(Sahih al-Bukhari, al-Buyu', Hadith: 2134)
(2)
Exchanging gold for silver or vice versa is called bay‘ sarf (currency exchange sale).
In this, immediate possession is a condition.
To defer (the exchange) is usury, which is why Umar radi Allahu anhu instructed Malik bin Aws radi Allahu anhu to take immediate possession.
(3)
Hafiz Ibn Hajar rahimahullah states:
From this hadith, it is understood that wheat and barley are separate categories, whereas Imam Malik and al-Awza‘i have considered them to be of the same category.
In one narration it is stated:
"Sell wheat for barley in whatever manner you wish, provided the transaction is hand to hand."
(al-Sunan al-Kubra lil-Bayhaqi: 5/277)
This hadith is authentic and explicit that these two are separate categories.
(Fath al-Bari: 4/478)

Hadith Commentary:

From this hadith, it is derived that barley and wheat are separate categories.

This is the view of Imam Shafi‘i rahimahullah, Imam Abu Hanifah rahimahullah, Imam Ahmad rahimahullah, and all the scholars of hadith.

Hadith Commentary:
The buying and selling of one currency in exchange for another currency is called "sarf" (currency exchange).
In the transaction of sarf, possession and hand-to-hand exchange are conditions.
If both counter-values are of the same kind, then taking more or less on either side is forbidden (haram), and this transaction must also be cash for cash.
Similarly, if the kinds are different, then an increase or decrease is permissible, but this buying and selling must be hand-to-hand.
In fact, Malik bin Aws radi Allahu anhu had some dinars and wanted to take dirhams in exchange for them.
His transaction was settled with Talhah, but Talhah radi Allahu anhu said, "At this moment I do not have dirhams with me; when my treasurer comes from Ghabah, I will provide you with the dirhams."
Umar radi Allahu anhu was listening to these matters; he said:
"Do not separate until you have received the dirhams from him, because the Messenger of Allah sallallahu alayhi wa sallam said:
'Exchanging gold for silver is usury (riba), except when it is hand-to-hand, then it is permissible.'"
And Allah knows best.

Explanation:
1:
Exchanging silver for gold,
and gold for silver with a difference in amount is permissible,
but "hand to hand" (immediate exchange) is the meaning of this hadith,
not that exchanging gold for silver with a difference in amount is not allowed.
See:
Hadith (No. 1240)

(1) In this hadith, the type of usury (riba) that is related to buying and selling is described. The second type of usury is that which pertains to transactions, i.e., stipulating the condition of receiving more in return for lending a small amount. This is called the usury of loans (riba al-qardh). In buying and selling, usury occurs when both sides involve the same commodity, but there is an increase or decrease (in quantity), or the transaction is deferred, i.e., not on the spot, as has been clarified with examples in the above narration. Or, the commodities may be different, but the transaction is deferred, as in the first example where it is explicitly stated that there is usury even in exchange of gold for silver if the transaction is not on the spot, because the prices of things and commodities keep changing. Therefore, whether both sides involve the same commodity or different commodities, there must absolutely not be any deferment; however, if the commodities are different, then an increase or decrease (in quantity) is permissible. If the transaction involves currency (rupees, money) with any commodity, for example: dates, wheat, barley, etc., then deferment is also permissible in this case.

(2) "Except on the spot" — in Arabic, the words are: إِلَّا هَاءَ وَهَاءَ, meaning both parties say to each other, "Here, take your goods." When both say this, the transaction will necessarily be on the spot; therefore, this meaning has been necessarily derived.

Benefits and Issues:
➊
Talhah bin Ubaydullah (radi Allahu anhu) is among the ten who were given glad tidings of Paradise (asharah mubashsharah).
Despite this, he was unaware of this particular issue until Umar (radi Allahu anhu) clarified it for him. Therefore, being a great scholar does not mean that there is no issue which he does not know, or that it is impossible for him to make a mistake.

➋
If a person makes a mistake, another person should inform him that the correct ruling is such and such.

➌
It is permissible to swear an oath for the sake of emphasis.

➍
It is permissible to use the wording of an oath when commanding someone to do something or forbidding them from something.

Benefits and Issues:
➊
If the type (jins) of edible items is the same but the varieties are different, then exchanging them with each other is permissible with two conditions.

(a)
The quantity on both sides must be equal. For example:
One sa‘ (measure) of dates can be exchanged for one sa‘ of another variety of dates, but it is not correct to take or give two sa‘s of dates in exchange for one sa‘.

(b)
The exchange must be on the spot (hand to hand), meaning that both parties should take possession of the items in the same sitting.

➋
The same ruling applies to gold and silver.
Gold must be exchanged for gold hand to hand and in equal weight.

➌
If the type (jins) is different, then disparity in weight and quantity is permissible. For example:
In the exchange of wheat for barley, or gold for silver, it is not necessary for the quantities to be equal. However, it is necessary that the exchange be immediate from both sides (spot transaction).

➍
If a person has a lower quality of wheat and wants to obtain a higher quality of wheat, the permissible method is that he sells his wheat for cash, and then purchases the desired wheat with that money.

Benefits and Issues:
➊ The exchange of gold and silver with each other is permissible on the condition that payment is made immediately from both sides.

➋ If this condition is absent, then the exchange of gold for silver is religiously prohibited.

➌ The statement of Sufyan ibn ‘Uyaynah rahimahullah, “Take heed,” was to draw attention to the fact that in the exchange of different commodities as well, some forms are prohibited; therefore, care should be taken regarding them.

Hadith Authentication: [الموطأ رواية يحييٰ بن يحييٰ 636/2، 637 ح 1370، ك 31 ب 17 ح 38، التمهيد 281/6، 282، الاستذكار : 1290 ● أخرجه البخاري 2174، من حديث مالك به ورواه مسلم 1586، من حديث ابن شهاب به و صرح بالسماع]
Jurisprudential Points:
➊ In transactions involving the same kind (of commodity), taking more or less is usury (riba).
➋ An authentic solitary report (khabar wahid) is a valid proof.
➌ In transactions involving the same kind (of commodity), deferred payment is not permissible.
➍ The noble Companions were imbued with the spirit of enjoining good and forbidding evil. Radi Allahu anhum ajma'in
➎ Sometimes, even a very great scholar may remain unaware of an authentic hadith.
➏ No one’s statement is a proof in opposition to an authentic hadith.
➐ Due to lack of knowledge, an ijtihadi (juristic) error can occur, for which the one exercising ijtihad is excused.
➑ There are many types of usury (riba).
➒ In order to prevent usury (riba), Islamic law has made meticulous arrangements.

Benefit:
In this hadith, some important rulings regarding buying and selling have been mentioned, the necessary details of which are as follows:
In the exchange of gold and silver with each other, immediate payment is a condition; otherwise, it is usury (riba). That is, in the mutual exchange of these two commodities, deferred payment is prohibited, whether the deferment is for a short period or a long period. Similarly, selling wheat for wheat, barley for barley, and dates for dates on credit is prohibited, and this is usury (riba).