What is the valid period under sharia for a down payment?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

A down payment is to buy something and pay part of its price. If the transaction is completed, then the paid amount of money will be regarded as part of the price. If the transaction is not completed, the seller will take this amount as a gift from the buyer. Abu ‘Abdullaah (Ahmad) said, “Down payment is that a man buys a camel for one hundred dinars, so he gives the seller two dinars as down payment and says, ‘If I do not buy the camel, the two dinars will be yours.’ It is said – and Allaah Knows best – that a man buys something and pays a dirham, or less or more, to the seller, saying ‘If I do not buy it, the dirham is yours.’” [Ibn Maajah]

Scholars did not mention a specified time – that is impermissible to exceed – for this sale under Sharee'ah. This period is subject to the agreement of the involved parties. After defining the down payment sale, the Hanbali scholar Al-Mardaawi said, “…whether there was timing or not…it is said that down payment is to say, ‘If I take the sold item and bring you the remaining money on such and such a date, otherwise it (the down payment) is yours.’”

Allaah Knows best.