If he buys an item and pays the price and has something left with the seller
Question

                Is it permissible for a seller to delay returning the rest to the buyer in the event of him having no change? For example the buyer gives a 100 rial bill to the seller, in order to buy goods worth 20 rials, and the seller does not have any change, at which point he agrees with the seller that he may delay returning the 80 rials until the next day or until after a short period after they part? What is the ruling if they do not part, such as if the seller goes to look for change or he goes to his safe inside the store?.
Praise be to Allah.
    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.