Qasim bin Muhammad says that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) used to dislike settling with a mukatab who owes gold and silver, whereby (the master) settles with him for a third or a quarter of it, or whatever it may be, and he would say: Make that in goods, on whatever terms you wish. Qasim says: Umar bin Abd al-Aziz (may Allah have mercy on him) wrote this to Abu Bakr bin Muhammad.
Shaykh Abu al-Walid (may Allah have mercy on him) said: Our companions said: We do not permit the master to take less than the dirhams in their place, because it is usury (riba).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21714