What should a person do who borrowed a sum of money from his uncle in a European country, and repaid part of it in monthly installments for two years after his uncle demanded it, then the uncle demanded an additional amount, claiming he forgot to add it, and the borrower is certain he did not receive this additional amount?

This amount is not established as a debt upon you except with evidence, and you are not obliged to repay it. The original principle is the freedom of one's liability from debts, and it is not engaged except with certainty. Al-Izz ibn Abd al-Salam said: "The original principle is the freedom of the servants' liability from rights until the causes of their obligation are certainly established." Al-Juwayni said: "Everything whose obligation is ambiguous, the original principle regarding it is the freedom of liability."