Question: Some of the people purchase goods from a seller and they have wealth left over that they leave with the seller until they return. Then the seller uses that money in his business until the buyer returns to take what he is owed?
Answer: There is no harm in this. It would be prohibited if it was an exchange of money for money (i.e., currency for currency). As for buying goods, then there is no prohibition in this.
Source: Fathul-Rabbil-Wudood fil-Fatāwa war-Rasāīl war-Rudood 2/68 of Al-‘Allāmah Ahmad ibn Yahyā Aali Shabīr An-Najmī (rahimahullāh).