Amr bin Dinar (may Allah have mercy on him) says: Sayyiduna Ibn Abbas (may Allah be pleased with them both) said that there is no harm in the debtor saying: "I will pay you before the due time, but you waive part of the debt for me."
A hadith with a connected chain (musnad) has been narrated about this, but there is weakness in its chain.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11135