Nasai

Such a sale in which gold is bought or sold in exchange for silver, or gold for gold, or vice versa—that is, silver in exchange for gold or silver for silver—is called bay‘ al-sarf (currency exchange transaction). In this, immediate payment (spot transaction) and equality (in weight/amount) are necessary, whereas in the mutual exchange of different items, the condition of equality is not required; however, immediate payment is still necessary in that as well.

Hadith Commentary:
That is, an increase or decrease in it is permissible, but the condition of hand-to-hand (immediate exchange) applies to this as well; it is not permissible for one side to be cash and the other on credit. And by gold and silver, what is generally meant is both coined and uncoined forms.

Hadith Commentary:
Gold must be sold for gold and silver for silver in equal amounts, and if the commodities are different—for example, gold on one side and silver on the other—then an increase or decrease (in amount) is permissible. However, deferred payment is impermissible; rather, it is necessary for both sides to be immediate (hand to hand). It is not permissible for one side to be immediate and the other deferred. Similarly, it is prohibited for both sides to be deferred. In any case, if the commodities are different, then an increase or decrease is allowed, but taking possession in the session of sale is a condition. Deferring (the exchange) is haram. The Shari‘ah has declared this to be riba (usury).

Hadith Commentary:
In this hadith, the restriction of "hand to hand" (immediate exchange) is not mentioned, but from another narration in Sahih Muslim it is established that "hand to hand," meaning immediate payment, is also a condition in this case. Among the scholars, there is consensus that in the exchange of currency (bay‘ al-sarf), taking possession (qabdh) is a condition.
The difference of opinion is regarding whether, when the type (jins) is the same, an increase or decrease (in amount) is permissible or not. The majority opinion is that it is not permissible.
And Allah knows best.

Hadith Commentary:
(1)
Imam Bukhari rahimahullah sometimes establishes a chapter heading and then clarifies the hadith that falls under it. At the end of this hadith, it is mentioned that we may buy gold in exchange for silver and silver in exchange for gold in whatever manner we wish. In this hadith, there is no stipulation of the transaction being hand-to-hand (immediate exchange).

(2)
Imam Bukhari rahimahullah, by establishing a chapter heading, has indicated towards some chains of transmission of this hadith in which the words of hand-to-hand (immediate) transaction are present. Thus, in Sahih Muslim it is stated that in the case of differing types (of commodities), you may buy and sell as you wish, provided it is hand-to-hand (immediate exchange). (Sahih Muslim, Al-Musaqat, Hadith: 4063(1587))

(3)
The aforementioned hadith discusses the sale of currency (bay‘ sarf), meaning that when one wishes to buy the currency of one country with the currency of another, disparity in amount is permissible, but deferred payment is not allowed. If you wish to buy Pakistani rupees in exchange for dinars, then at the time you give the dinars, you must receive the rupees at that very moment. If there is any delay from either side, Islam considers this to be usury (riba). This is a common observation nowadays that the exchange rates of currencies and the rates of gold and silver change moment by moment; if the exchange is not immediate and there is a delay in receiving the other item after giving one, the rate will have changed. The same ruling applies to the exchange of basic food staples with one another, that disparity in amount is permissible, but the transaction must be hand-to-hand (immediate); it must not be deferred.

Hadith Commentary:

Benefits and Issues:
➊ In the mutual exchange of gold and silver, an increase or decrease (in quantity) is permissible,
but it is necessary that the transaction be hand to hand (spot payment).