Barcode scan for different actions

I have 2 tables. One with products, the other with the actual inventory or Supplies I have in my house (the app is for private use). I have made the barcode the key for both tables. It is set to be scannable. Now, when I scan a pre-existing barcode the only option is to add a new row to the table. How could I make it so that I can either add this new row, add to the number of items in an existing row OR diminish the same number?

@Marc_Engel
Can you please elaborate what do you mean with “… How could I make it so that I can either add this new row, add to the number of items in an existing row OR diminish the same number?..

I would like to be able to add 1 item in a ‘Total number’ column of a specific product that is in my Supplies table. In the same barcode scan I would like to have an option to the opposite, namely distract 1 from the ‘Total number’ column of the row that my barcode has helped identify. And if the product doesn’t exist (i.e. A new barcode number) I should be able to add this product to my product table and proceed to add a row (with 1 item or more in the Total number column) in my Supplies table. Normally I could use the standard plus icon that AppSheet provides me with, but the first two actions will not be permitted this way. I guess I am asking how to construct one or several actions that give me this possibility. But as I understand that would leave me with 3 instead of 1 icon AND I would have to know before clicking whether this product already exists, or not. Hope I am making more sense now?