What is the ruling of Islamic law regarding the increase requested by the gold shop owner in exchange for the buyer's delay in paying the remainder of the amount, after they had agreed to pay the full amount at the daily gold price upon the delayed payment?

Selling gold for banknotes requires immediate exchange (qabd) at the contract مجلس (session). If one of the two counter-values is delayed, the contract becomes invalid (fasid) by scholarly consensus. An invalid sale does not transfer ownership, nor does it entail any legal effect, and the sold item must be returned. If a portion of the price has been received, the contract is nullified for the unreceived portion, but not for the received portion, according to the majority of scholars. The buyer is obliged to return the gold invalidated in proportion to the invalid amount. If returning the physical gold is not possible, its value on the day of performance may be returned.