Is it permissible for the seller to delay returning the remaining amount to the buyer when change is unavailable, and does the ruling differ if they do not part ways?

What the buyer leaves with the seller due to a lack of change is considered a trust (amanah), and there is no harm in that. And if the buyer goes to fetch the remaining amount from inside the shop or from a neighboring shop, there is no harm in it, because it is not considered an exchange (sarf) which requires immediate قبض (qabd - mutual possession), but rather it is a distinction of right and a return of money to its owner. The Permanent Committee for Issuing Fatwas was asked about a buyer leaving some money with the seller and replied: "There is no usury in the buyer leaving some of his money with the seller, because this falls under the category of sale and entrusting the seller with the remainder of the price, and not under the category of exchange (sarf)."