What is the ruling on receiving an increase in the price of gold if the buyer delays payment and the price of gold rises?

It is not permissible to sell gold for money on a deferred basis, neither in full nor in installments. Rather, possession must take place in the contract session; otherwise, it would be the prohibited usury of nasi'ah (deferment).

If the sale is deferred, it is not religiously permissible, and the gold remains the property of the seller.

If something other than gold is sold on a deferred basis, it is not permissible to take an increase on the original price for delayed payment. Instead, the solvent debtor is obliged to pay, while the insolvent one is given time until he becomes able to pay.