Bukhari

Hadith Commentary:
➊ If the sale of goods is conducted with other goods, this is called barter (muqayadah).
➋ If the sale of goods is conducted with cash, then the cash is called the price (thaman) and the goods are called merchandise (‘ard).
➌ If cash is exchanged for cash, but of the same kind—such as gold for gold or silver for silver—this is called equal exchange (muraṭalah).
➍ If the types differ, such as exchanging silver for gold or vice versa, this is called currency exchange (ṣarf).
➎ In ṣarf, disparity in amount is permissible, but immediate exchange (i.e., hand-to-hand transaction) is necessary and obligatory, and delaying possession is not permissible.
➏ In muraṭalah, both equality in amount and immediate exchange (hand-to-hand) are necessary.
➐ If the sale is between price (thaman) and merchandise (‘ard), it is permissible to set a term for either the price or the merchandise.
➑ If the term is set for the price, it is a loan (qard); if the term is set for the merchandise, it is a forward sale (salam); both are permissible.
➒ If a term is set for both, it is a sale of deferred for deferred (bay‘ al-kāli’ bil-kāli’), which is not permissible.
(Wahidi)

Hadith Commentary:
(1)
Generally, buying and selling are conducted through currency, but in the terminology of the jurists (fuqaha), the types of buying and selling are as follows:
٭ Muqayadah:
Purchasing other necessary items by giving grain, cotton, etc.
Even today, in some rural areas in the early period, items are bought from shops by giving wheat and the like.
٭ Muratala:
Buying and selling one currency in exchange for the same currency is called muratala.
The condition for this is that the amount exchanged must be equal on both sides and the transaction must be hand-to-hand.
٭ Sarf:
Buying and selling gold for silver or one currency for another currency is called sarf.
In this, mutual difference in amount is permissible, but deferred payment is not allowed.
For example, in exchange for one hundred grams of gold, several hundred grams of silver, or in exchange for one riyal, several rupees can be bought, but it is not permissible to buy new notes of a national currency for old notes with a difference in amount.
٭ Purchasing necessary items with cash—such cash transactions are always ongoing.
In this, deferred payment is also permissible.
If the price is paid in advance and it is agreed that the item will be received later, this is called salam, and doing so is permissible with certain conditions.
Similarly, taking the item and paying the price later is also permissible, but it is not permissible for both the price and the item to be deferred.
This cannot be called bay‘ salam either.
In any case, the ruling for all these types is that hand-to-hand transactions are all permissible, but there are some details regarding deferred transactions.
It is not permissible to defer payment in exchange for payment (i.e., currency for currency), but it is permissible to defer payment of goods in exchange for payment.
If payment is received and the goods are to be delivered later, this is also permissible.
If both sides are deferred, then this is not permissible in any case.

(2)
Imam Bukhari rahimahullah established a chapter regarding buying and selling silver for gold on deferred payment.
It is proven from the hadith that doing so is not permissible.
There may be a difference in amount, but the transaction must be hand-to-hand.