What is the ruling on someone who donates a fixed amount monthly, then gives another amount with the intention of charity on behalf of himself and the one who gave him the money?

Persevering in giving a fixed amount of charity monthly is a righteous and beneficial act. If someone gives you money to donate as charity, then dispense it to those who are deserving or according to the owner's condition. If the intention is that you took the owner's money and intended it as charity for yourself and for him, this will not be counted for you but only for the owner, and you will receive the reward for assisting. Giving a fixed amount as charity is a voluntary act, and it is best not to interrupt it, for the most beloved deeds to Allah are those that are most constant, even if they are small.