What is the solution when the shop owner forgot to inform the buyer about the difference between the cash payment price and the deferred payment price for the goods, which he agreed to pay later?

The sale is concluded at the agreed-upon price. However, if the seller proves his mistake in the price (by distinguishing between an immediate sale and a deferred one), or if the buyer confirms it, then it is permissible to annul the sale and reclaim the item, then sell it at a new price. Nevertheless, it is preferable to proceed with the sale as it was concluded if the deferral period is short and does not typically affect the price, as this avoids dispute. If the buyer does not fulfill the deferred payment, the seller has the right to annul the sale. It is not permissible to increase the price in exchange for delayed payment after the sale has been concluded, as this is usury (riba).