Why does my billing show a negative amount?

Hi gurus,

I have a problem with my SAP report, payment should not be a negative figure, but in this report it is a negative figure. and i just can't find the reason for this. it is because of the document type? anyone can help me? or any way for me to check?

I am fairly new to SAP and have a lot to learn.

Thank you in advance,

Bella.