Hadith.3472 - Ali ibn Ja'far asked his brother Imam Musa ibn Jafar Al-Kadhim (as), about a man who entered into a mukataba (a contract of emancipation) with his slave.
After the contract, the slave said: "Forgive me part of my payments, and I will pay off the remainder early."
Imam (as) said: "If it is given as a gift, then there is no harm. But if he says, 'Reduce my payment in exchange for early payment,' then it is not permissible."