Lookup Matching Data & Subtract From Another Lookup

I have a very large spreadsheet of customer information(I call it the master spreadsheet). Each row contains only 3 things: Account number, product bought, Price

Later I receive the money from the customer for that product(the pay sheet) that contains the exact same thing in the same order: Account number, Product bought, Price paid.

What I'm trying to do is compare the two spreadsheets so that when i receive the pay sheet of cusomters who have paid with the amount it will deduct it from the master sheet.

So it should compare account numbers when it finds a match then it should subtract the amount paid (column C) from the master spreadheet price column(column C also).

sometimes customers don't pay the right price so it has to be a subtraction so I can see if it was over paied, underpaid etc.

Right now I'm still doing it manually combining the two documents sorting it by account number and checking for matches in column A (account number).