Is it permissible to take the difference in the exchange rate between the British Pound and other currencies (Euro or Dollar) when selling goods, based on the seller's determination of this exchange rate?

If the seller is a sales employee, he is not permitted to take the exchange rate difference for himself; rather, it belongs to the company.

However, if the seller trades with his own money, there is no objection to him taking the exchange rate difference if the commodity is sold in one currency and its price is received simultaneously in another currency at an equivalent value. But if payment is delayed, it is not permissible to agree to this at the time of the contract, because it is a currency exchange (sarf) which requires immediate mutual possession (taqabud).

Yet, if the seller sells his commodity in Egyptian Pounds, and the buyer pays in US Dollars or another currency when the payment is due, there is no objection, provided there is no collusion.