To what extent is it true that if a seller leaves the gathering because he does not have change, and then returns to deliver the remaining amount to the buyer, this is considered Riba An-Nasi'ah (interest due to delay)?

If a person buys a commodity and pays an amount greater than its price, the excess is a trust in the hands of the seller. This is not usury, because there is no exchange of money for money. It is permissible for them to separate before distinguishing the excess. Then, the buyer may take the remaining amount, or buy something with it, or make it a loan, or gift it to the seller.