There is much more to database and stock control than
...reduce a quantity of an item, say 8, and order 3 of those items....
Do you have a complete description of the business and associated rules? That's your starting point.
I notice you have 2 posts on this forum, could you please tell us more about yourself and your background, so more focused and appropriate responses could be made.
Reorder point/level, backorder, substitution may all apply, but we don't know the details of you or your "business".
Usually, people deal with transactions, where, for each product/StockItem
Code:
CurrentStock = LatestStockTakingAmount +StockAcquired - StockSold
and
StockAcquired are positive transactions and StockSold are negative transactions. This allows you to do arithmetic on the transactions for a stockItem and determine current levels.
You may get additional info from Allen Browne's article.
Good luck with your project.