Sum number on specific date

[distro]

Hi!

I got several sheets representing clients and 1 main sheet as an overview of my clients.

Im trying to create a stock sheet that updates itself as I input Kg sold in the customers sheet.

So, what I want is as follows:

Subtract B:B(client sheet) from B2(stock number) IF A:A(Client) >= A2(stock input date)

B:B represents kg in numbers
B2 represent current stock available kg in numbers
A:A represent dates the product was sold
A2 represents the date where the stock was updated

I have tried sumif (which works on other formulas I got), but since I don't really understand all these formulas, I really need some help.

Any advice is highly appreciated