Why i am getting negative amount in already paid invoices?

I know what is happening here, info1401.

The only reason why we have a negative amount on a paid invoice is that the current amount is lesser than the original amount. This means that the invoice was edited.

We can review the Audit Log to who edited the invoice and what is the original amount. You can follow the steps in this article to easily trace it: How to view transaction changes in the audit history.

After that, we can already correct the amount when we go back to the invoice.

You can always get back to us if you have additional queries regarding the invoices.