A:
When the buyer leaves some of his money with the seller, this is not a form of Riba.
(Part No. 13; Page No. 181) This is a form of sale and involves entrusting the seller with the remaining money that is owed; it is not giving him an extra payment over the original price.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.