Is it permissible to buy and sell gold dental inlays/onlays in installments or with the possibility of exchanging them, taking into consideration their fixed price and the seller's offer of "buy two pieces and get the third for half price"?

Gold is not to be sold for gold unless it is like for like, hand to hand. Nor is it to be sold for currency unless hand to hand. If the sale is conducted correctly and the buyer takes possession of the gold, and then wishes to exchange it for other gold, a new contract is necessary, and equality in weight must be ensured. This is based on the Prophet's (peace be upon him) saying: "Do not sell gold for gold unless it is like for like, and do not prefer one part over another..." It is impermissible for men to use gold teeth for adornment, though it is permissible out of necessity.