What is the ruling on selling chicks by taking a down payment and determining the price at the time of delivery based on the market price of that day, knowing that the seller does not own the chicks at the time of taking the down payment, and that the chicks spoil if not sold on the same day?

If the seller does not own the chicks, then what he takes is not considered a down payment (urbun), but rather a seriousness margin (hamish al-jiddiyah). The latter is absolutely prohibited by the majority of scholars, and this is the most preponderant and adopted opinion. As for determining the price based on the stock market price on the day of delivery, there is no objection to it if the sale contract will only be concluded at the time of delivery, and both parties have the right to proceed or cancel, and what preceded it is merely a non-binding promise. To correct the sale before the due date, the chicks can be sold as a forward sale (bay' salam) — based on the preponderant opinion that a forward sale of animals is permissible — provided that they are described with a precise description that removes ambiguity, and the price is agreed upon and paid in full at the time of the contract, with the delivery time specified. This is what the majority of jurists have adopted.