Is the additional amount paid to the seller in exchange for returning an item of the same type after its price has increased, which the seller had sold to another due to its spoilage, considered usury (riba)? And is it the right of the buyer or the seller?

As long as the forgotten item with the seller is of a type that spoils quickly, his disposal of it by selling is valid. If he sold it for the benefit, you are only entitled to its price. Whatever transpired between you after that is considered a new sale at the agreed-upon price, and there is no usury in it. Therefore, you must return the excess amount you took from him.