Is it permissible to take a commission when exchanging coins for banknotes?

If the contract is for metallic currency, and he takes an increase of 300,0 for every million, this is permissible, unless the price is paper currency. If it is paper currency, then the increase is forbidden usury (riba), and the contract is void, because Allah Almighty says: "O you who have believed, fear Allah and give up what remains [due to you] of interest (riba)." However, if he is appointed as an agent to sell the metallic currency and takes a fee for that, there is no harm, based on Al-Buhuti's statement that agency for a known fee is permissible. He is also allowed to exchange for himself from paper currency if he exchanges it (the metallic currency), and this is acceptable. But he is not allowed to exchange for himself and take a fee, because that is a trick to conceal usury.