What is the ruling on buying gold and paying its price, then returning it due to its unsuitability in terms of size or type, with the condition of paying or receiving the difference in price?

If the condition mentioned refers to the option of condition (khiyar al-shart), then this is void according to the majority of scholars, and valid with the condition being void according to the Hanbalis, because it compromises immediate possession (taqabud).

If the intention is to sell the gold you recently purchased, there are two scenarios: 1. Selling it at its market price on that day: This is permissible, provided that immediate possession occurs at the contracting session, based on the saying of the Prophet (peace be upon him): "Gold for gold... hand to hand." 2. Exchanging it with payment of the difference: This is not permissible because it is a usurious sale with a differential (riba al-fadl), based on the saying of the Prophet (peace be upon him): "Do not sell gold for gold unless it is like for like, and do not prefer one part over another."

The permissible solution according to Sharia is to sell the gold at its market price on that day and then purchase whatever you wish with its proceeds, as mentioned in the Prophet's (peace be upon him) hadith regarding the sale of dates of varying quality (al-tamr al-janib).