Customer 1 owes you $1,000 and this is the Dr balance on their receivables account.
They pay $1,000, but the fraudulent staff member pays only $800 into the company bank account and diverts the other $200 to themselves.
So that Customer 1 does not complain that they paid $1,000 but $200 is still shown as owing, when Customer 2 pays (say that customer owes $1,500), $1,300 is credited to Customer 2’s account and $200 credited to Customer 1’s account…..an so on. The staff member frantically tries to use new receipts to conceal the original theft.