Hi,
I am developing retail Point of Sales System. I have a question that, How should I manage stock. Should I keep my stock status base on invoice basis or summary basis

Invoice Basis
Every item from each invoice will be recorded in table which store the current item's qty. Therefore, item will be sold base on invoice or FIFO basis, all return, transfer, and etc have to refer to invoice no and supplier no.

Summary
One record per item.

How would u design it in real world? Is FIFO here is very important? DO you think design it in invoice basis is too troublesim.complicated for user and also developers? What do you think about both method, or do you have other suggestions?

Thank you
Pls advice