Hadith.3750 - Wahb ibn Wahb narrated from Imam Jafar ibn Muhammad Al-Sadiq عليه السلام from his father, Imam Muhammad ibn Ali Al-Baqir عليه السلام (peace be upon them), that Imam Ali ibn Abi Talib عليه السلام used to say:
"Earnest money (down payment) is not permissible unless it is part of the price."