Is it permissible to drop a percentage of the interest resulting from an Islamic Murabahah sale upon settlement of the remaining installments?

Waiving a portion of a debt can fall into one of two scenarios: The first is when the debt is due. In this case, there is no harm in waiving a portion of the debt. The second is when the debt is not yet due, and the creditor asks the debtor to pay in exchange for waiving a portion of the debt, or the request comes from the debtor. This is not permissible according to the majority of scholars and is considered usury (riba), known by the rule "accelerate payment and reduce the debt" (da' wa ta'ajjal). However, if the debtor comes to pay the full debt and the creditor tells him, "I have waived such-and-such amount from it, so pay the rest," this is permissible, provided there was no prior collusion.