What is the ruling of Islamic law on the system of deducting a portion of the price of goods for the highest consuming merchants at the end of the year, and is this considered a sale with a condition, or does it have a legitimate Islamic justification?

The scenario mentioned does not fall under the category of a valid sale and purchase contract, and its invalidity stems from its inclusion of gharar (excessive uncertainty). The price of the goods is not known at the time of the contract, which leads to price instability. This invalidates the sale due to the unknown price. The Prophet (peace be upon him) forbade two sales in one sale.