What the buyer leaves with the seller if he has no change is regarded as a trust and there is nothing wrong with that, whether they agree that he will take it after a short time or a long time.
It says in Kashshaaf al-Qinaa’ (3/269): If he bought silver for one and a half dinars, and the buyer gave the seller two dinars so that he could take the price from it, and the seller took the price from the two dinars, even if they part, then the transaction is still valid because the hand to hand exchange took place before parting, and what is delayed is separating his right from that of his counterpart. The excess amount from the two dinars is a trust held by the seller. End quote.
The scholars of the Standing Committee for Issuing Fatwas were asked: I am the owner of a grocery store and I have a problem when I am selling. Sometimes the customer comes and buys something, and he gives me money and I owe him something. If I do not have any change, and I owe him something, he says: Tomorrow I will come and take the rest. For example, if he buys 50 rials’ worth and gives me 100, and I do not have 50 rials, so he says, Keep it with you until another time. Some people have told me that this is a kind of riba but I am not able to convince the buyers. I hope that you can give me a written fatwa urgently, so that I will understand.
They replied: The purchaser’s leaving some of his money with the seller does not involve any kind of riba, because this comes under the heading of selling and entrusting the seller with the rest of the money, it does not come under the category of money exchange.
And Allaah is the source of strength. May Allaah send blessings and peace upon our Prophet Muhammad and his family and companions. End quote.
Standing Committee For Academic Research And Issuing Fatwas
Shaykh ‘Abd al-‘Azeez ibn ‘Abd-Allaah ibn Baaz, Shaykh ‘Abd al-‘Azeez Aal al-Shaykh, Shaykh ‘Abd-Allaah ibn Ghadyaan, Shaykh Saalih al-Fawzaan, Shaykh Bakr Abu Zayd
Fataawa al-Lajnah al-Daa’imah (13/180)
From this we may know the ruling on the second issue, which is the seller going to bring the rest from inside the shop or from a neighbouring shop: there is nothing wrong with that, because this is not like money exchange in which it is stipulated that there be a hand to hand exchange; rather it is deducting one’s due and returning the rest of the money to its owner.
And Allaah knows best.