What is the religious ruling regarding the debt owed by the debtor to his deceased mother, who acquitted him of the debt before her death for an amount less than the actual debt that the debtor knew about, and what should he do about that?

If a mother waives her son's debt during her death sickness, it is considered a bequest to an heir and is only valid if the other heirs permit it. However, if the waiver occurs when she is not in her death sickness, and all heirs acknowledge or consent to waiving their right, then there is no obligation on the son. But if the heirs do not acknowledge or consent, then the son must return their shares of the debt.