What is the ruling on giving money to an expert to purchase foreign currency, with the sum being handed over today and the currency and a portion of the profit being received tomorrow?

This transaction is categorized as the expert acting as an agent for the owner of the money in carrying out the exchange (sarf), and agency in exchange is permissible by consensus (ijma'). Agency is valid for a fee that is a percentage of the profit, which is the madhhab of the Hanbalis.