Is the amount that the uncle will pay monthly in repayment of a debt considered unlawful (haram) or a form of interest (riba), given that a customary agreement stipulated this?

If an uncle bought a piece of land from the father and did not pay its price, and a document for this debt was written in the name of the questioner, and the uncle procrastinated in payment, then they agreed to pay a monthly sum in exchange for postponing the debt payment, it is not permissible to take any amount in excess of the debt in exchange for postponement; because it is prohibited usury (riba), as stated in the resolution of the Islamic Fiqh Academy, which prohibits stipulating any increase on a deferred debt. However, if the debt amount is paid in installments without any increase, then this is permissible.