Update stock using Zapier


I have a POS base where one record = one item sale (and i can define the quantity of that item that i sold).
For example: Product = Blue pen; Quantity = 3

Then, there is an Inventory base with a Products table where all the products are listed with the corresponding Sales and Stock.

I need to create a zap that updates the Sales in the Product table (has to add the multiple individual sales from the POS base), so that I get the Stock number right.

How can I do this? I’m trying a multi-step zap New record (in POS base) > Find (corresponding product in the Products table) > Update (the Sales in the Products table), but the updated Sales record just replaces the previous number, and I need it to add the quantity to the previous number…

How can I do this? Is there a way to tell Zapier to add the number instead of replacing it? What am I doing wrong?