To what extent is the guarantee offered by stores for a limited period on products in exchange for an additional sum permissible?

It is permissible for the seller to guarantee the merchandise to the buyer against defects and faults for a specific period. This falls under the "option of condition" (khiyar al-shart), which was legislated to avert anticipated harm. It is permissible for the seller to specify two prices for the merchandise: one with the option of condition and the other without it, provided that the contract is concluded based on only one of these prices, and the specification occurs before the contract as a form of bargaining.