Is the statement of a relative sufficient to consider money sent as a debt that must be repaid, and what is more befitting in this case, and is one rewarded for returning this money?

It appears that the man gave you a choice between taking the money as a gift or as a loan. It is better for it to be a loan so that you are not indebted to him. You must be keen to repay it if it becomes easy for you, as repaying a debt is a religious obligation, and obligations are the most beloved things by which a servant can draw closer to God.