Leaving the price of gold with the buyer as a trust after selling it

Q 1: Sometimes when I buy the old gold from a client, he prefers not to receive the money and says: Leave it as deposit and when I buy the new gold, I will deduct the price and give you the rest. Is it permissible for me to keep the sum which is the value of the old gold (Part No. 13; Page No. 479) until he buys the new one, at which time I give him the rest or deduct if I have something left?

A: If the case is as you have mentioned, it is not permissible because the condition of selling gold for silver or other kinds of the same value is that they should be delivered hand in hand.