What is the meaning of agency in buying and selling, especially concerning currencies?

Agency is the authorization for another to act on one's behalf. It is permissible according to the Qur’an, Sunnah, and scholarly consensus. Among its proofs is the Almighty's saying about the People of the Cave: {So send one of you with this silver coin of yours to the city}, and the Prophet's (peace be upon him) appointment of Urwah al-Bariqi as an agent to buy a sheep for him. The Ummah has reached a consensus on its permissibility due to the need for it. Agency is valid in currency exchange (selling currencies) if the exchange takes place in the same sitting.