Adding value to another sheet when creating a new entry in one table

[Guillaume_Guita]

I’ll try to make it simple:

I have two tables. On is an inventory for all SKU in our store. The other is a request for product transfer from a supplier

So table one: SKU, qty in stock
Table two: SKU, requested qty, request status

Once the request is processed, I need one action that will:
Add the requested qty to whatever qty is already in stock in table 1
If the sku is not already in table 1: create it and add requested quantity.
Change the request status to “transfered”

How do I proceed to do this?