Hi there,
I will be happy to help you with that.
We can use Google forms to achieve so, we can create one form where the user can choose the direction, in or out, then the product and the amount, then a Google Script will fire, read the amount added or subtracted and adjust the data in the inventory sheet.
The form will be static however, this is the disadvantage here, if you added a new product or removed a product, you will have to adjust the form accordingly.
There is another solution using Google Apps Script web apps, that will overcome this disadvantage, but will cost you a little bit more.
I will be happy to address any of your questions!
Sincerely,
Mohamed