Buyer opts to pay on a specific day

Question

What is the rule regarding the contract between two persons where the buyer agrees to give the money in credit and opts to pay on the first day of summer or winter or on the day of the Christians fast or the Jews fats ending?

zd · Answer

This is a Faasid sale as the time for payment is not exactly known. [NB: If the exact date of the stipulated day is known then the sale will be valid] (Kitaabul Buyoo).