As a simple rule, it's known to everyone that a negative amount cannot be applied to a negative amount in NAV or Business Central.
It means that we cannot apply a Sales Credit Memo to a Customer Payment (Payment received from the customer).
But, Let me tell you that it's not completely true. We can apply Sales Credit Memo to a Customer Payment (Payment received from the customer) despite of both having -ve amounts.
Let's find out how it's possible by watching this small video.
Hope it helps! Cheers