Is the money my maternal uncle paid a settlement of my mother's debt or a gift from him?

If there is no proof that your mother lent money to your maternal uncle, and the uncle swore by Allah that he did not take anything, then his word is to be believed, based on the hadith: "The burden of proof is on the claimant and the oath is on the one who denies." And based on the hadith: "Whoever swears by Allah, let him be truthful, and whoever has an oath sworn to him by Allah, let him be satisfied."

Accordingly, there is no blame upon you for accepting the gift, relying on your uncle's oath.