What is the ruling on selling foreign currency at a pre-agreed price, with its value delivered to another party in the recipient's country, and then selling it to a third person for a profit margin, out of fear of falling into usury of delay (riba an-nasi'ah)?

The transaction between you and your maternal uncle is an exchange (sarf), and it requires immediate possession (taqabud). Delay is not permissible except in cases of an urgent necessity that cannot be avoided.

The method of achieving immediate possession, if your mother is your uncle's agent, is for you to hand over the amount to your mother at the very same time the amount transferred by your uncle enters your account.

It is not permissible for you to be both a party to the exchange with your uncle and his agent in taking possession simultaneously. This is the view held by the majority of scholars.

If the exchange is conducted according to its legitimate rulings, then it is permissible to sell the amount you purchased at any price, even if it is for more than what you bought it for.