Is it obligatory to pay the seller the amount again after a portion of the debt was waived for prompt payment, due to ignorance of the fatwa that prohibits this?

Since the issue is subject to scholarly disagreement, it is permissible to uphold past instances where a portion of a debt was waived in exchange for immediate payment of the remainder, as this does not constitute usury (riba). However, it is preferable to pay the full amount as a precautionary measure. Although the preponderant opinion is that it is impermissible, many scholars have sanctioned acting upon a less favored opinion after the fact.