Is the aforementioned transaction permissible, considering the buyer's repeated changes of mind and promises, his delay in payment, and his attempt to alter the sale price and payment method after the initial agreement?

Such a sale is invalid unless one of the two prices is specified at the contracting session. Otherwise, it becomes void because it falls under "two sales in one sale," which is forbidden by Islamic law. If the contracting session concludes with a specific price, the sale is valid, and the buyer is obligated to pay that price. However, if it concludes without specification, the sale is void and must be annulled, with the price returned to the buyer and the goods to the seller. If the goods are damaged while in the buyer's possession, their value becomes due to the seller, with a difference of opinion regarding the day for determining the value (day of possession, day of loss, or the highest value). A void sale does not confer ownership, and whatever the buyer takes possession of is guaranteed by him at its value on the day of damage.