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