Relationship between stock and product
So a single product can potentially have multiple prices at once? If this is the case then I don't see the need for the extra table. Create the products table, including product name, description plus whatever else you need. Then create the stock table, with a FK relationship to the products table product ID with quantity and price per unit. When a sale is made it can take the price from the stock table and enter it as a sale price in the sales table and update the quantity in the stock table. 

The only thing that isn't clear is where you said a stock can have multiple products. The above seems to be a better method of handling this rather than having a product with multiple stock records and stock records with multiple products.