You can create a stock location for you customer an sell the products out of this stock. You have 500 pieces of "whatever" in stock "name of the customer". You can create it as a view or a real location as it fits to your needs.

thanks for your reply the problem is when i create the main order, it put in demand for the total of the order, whatever the scheduled date. so if the main order of my customer is planned along the year, i can't sell the same product to another customer untill the whole manufacturing is done.

so the stock is too hight !

regards didier