As long as InvoiceID is a number, wouldn't it be
Nz(DSum("Amount", "PaymentsT", "InvoiceID = " & InvoiceID & ""),0) ?
I'm not sure you can use Me in a domain lookup criteria.
EDIT: After testing in immediate window, I see that the double quotes are not needed after the control concatenation. Interesting. So maybe the Me works? I will have to test that.
EDIT2: OK, so elsewhere that I read, they lied. It does work either way.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.