Must a debt, which was promised to be waived upon success, be repaid if it is demanded after the condition has been met?

The friend's statement, "If you succeed, this amount is a gift for your success," is a conditional gift or a conditional release from debt, and it is considered valid by some scholars, such as Sheikh al-Islam Ibn Taymiyyah. If success is achieved and the money is received, the giver is not allowed to retract his gift, for "one who retracts his gift is like a dog that returns to its vomit." It is not permissible for the donor to retract his gift, except for a father regarding what he gives to his child. Your friend is not allowed to retract the gift, and you are not obligated to return the amount. You should inform him of the Sharia (definition) ruling. If he insists on the request, it is commendable to give him what he requested while informing him of the sin, or to give him something else to reciprocate the gift.