Lending someone money and bringing it up every chance you get turns a favor into a leash.

Money handed over with a reminder attached is neither a gift nor a loan, it is a hold on somebody. Each mention resets the debt and quietly places the other person lower in the friendship, which they feel clearly even when nothing is said. Whatever the amount was, the friendship now costs more than that. Generosity with a string in it is a slower form of buying.

Decide before you lend whether you could afford never to see it again. If not, say no, which is a completely acceptable answer. If you do lend it, mention it once and then leave it alone.