Hi there, Nicole_P.
I can help you share some insights about Sync with Square app.
This is a one-way integration that imports your transactions from the Square into Quickbooks but not from QuickBooks into the Square. With that said, I recommend contacting Square support team so they can check if there's a way to get the quantity of the product sold be reflected in Square.
You may check out this article for more information about Sync Square with QuickBooks Online
If you have any other concerns, please don't hesitate to add a comment below.