Nafi' narrates from Ibn Umar (may Allah be pleased with them both) that he used to say, regarding a person who makes a contract of manumission with his slave for gold and silver payable in installments, that he disliked that he should say: Give me such-and-such quickly, and whatever remains is yours.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21711
It is narrated from al-Hasan and Muhammad bin Sirin (may Allah have mercy on them both) that they both disliked, regarding the mukatab, that it be said: Pay the amount quickly and I will reduce the price.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21712
Ibn Abbas (may Allah be pleased with them both) said that if it is said to a mukatab: Pay quickly and I will reduce (the amount), there is no harm in it.
Shaykh Abu al-Walid (may Allah have mercy on him) said: (To say) Pay quickly whatever you wish, and for that I will free you and cancel the contract, there is no harm in saying this.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21713
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