Is it permissible for a colleague to exchange foreign currency for us into local currency, where the price is agreed upon in advance, and delivery sometimes takes place on the same day and sometimes on the following day?

The exchange of currencies with one another, known as "currency exchange" or "sarf," requires immediate possession in the same session when exchanging one currency for another. This is based on the Prophet's (PBUH) saying: "Gold for gold, and silver for silver, hand to hand, like for like, and any increase is usury (riba). If these types differ, then sell as you wish, provided it is hand to hand."

Accordingly, it is not permissible to delay the receipt of both currencies, or one of them, or a part thereof, beyond the session of the contract, otherwise, it falls into riba al-nasi'ah (usury of delay).

If your colleague takes the foreign currency from you, then goes to the merchant and sells it to him for local currency and takes possession of it from him, then transfers it to you via exchange offices, there is no harm in that, provided that the agreement with the merchant on the exchange rate is not binding at a later time.

However, if your colleague sells the foreign currency to the merchant before taking possession of it from you, this is not permissible due to the lack of the stipulated possession.