All perfect praise be to Allah the Lord of The Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.

We should differentiate between two situations:

First: In case the transaction is money exchange contract in the sense that a person wants to exchange a fifty-JD bill with smaller bills, then the payment must be made on the spot and it is impermissible to delay that even for a short period to avoid falling into usury. In addition, the big bill must be exchanged with smaller bills of equal value.

The evidence on this is the following narration: Narrated Abu Al-Minhal:

I used to practice money exchange, and I asked Zaid bin 'Arqam about it, and he narrated what the Prophet said in the following: Abu Al-Minhal said, "I asked Al-Bara' bin `Azib and Zaid bin Arqam about practicing money exchange. They replied, 'We were traders in the time of Allah's Messenger (PBUH) and I asked Allah's Messenger (PBUH) about money exchange. He replied, 'If it is from hand to hand, there is no harm in it; otherwise it is not permissible." {Bukhari No. (2061)}.

Moreover, Narrated Ibn Shihab: that Malik bin Aus said, "I was in need of change for one-hundred Dinars. Talha bin 'Ubaidullah called me and we discussed the matter, and he agreed to change (my Dinars). He took the gold pieces in his hands and fidgeted with them, and then said, "Wait till my storekeeper comes from the forest." `Umar was listening to that and said, "By Allah! You should not separate from Talha till you get the money from him, for Allah's Messenger (PBUH) said, 'The selling of gold for gold is Riba (usury) except if the exchange is from hand to hand and equal in amount, and similarly, the selling of wheat for wheat is Riba (usury) unless it is from hand to hand and equal in amount, and the selling of barley for barley is usury unless it is from hand to hand and equal in amount, and dates for dates, is usury unless it is from hand to hand and equal in amount" {Bukhari No. (2139)}.

An-Nawawi says, "According to scholarly consensus, it is forbidden to sell gold for gold on delayed terms and the same is true for wheat and barley, and every two items that share the Illah (Legal cause) of usury (Riba)." {Shareh Muslim, V.10:P.11}. 

Second: In case someone bought some products and paid for them but the seller didn`t have the needed change, so he told the latter that he would pick up that change later, then this is permissible as stated our (Shafie) jurists. In this case, the remaining amount is considered a trust in the possession of the seller. An-Nawawi (May Allah have mercy on him) said, "Our fellow scholars said: "If a person had ten Dirhams and another had one Dinar that is equal to twenty Dirhams and the former wanted to buy half the dinar from the latter, then this is permissible in the sense that the remaining half becomes a trust in the buyer`s possession."{Al-Majmou`, V.10:P.147}. And Allah the Almighty knows best.