Manager follows the standard double entry accounting rules where each transaction has a debit entry and a corresponding credit entry.
Your report shows both entries but obviously for the debit entry no amount is shown as you have not selected the debit column but only the credit column.
You need to use the where clause to select one of the transactions but not both.
Writing reports without context of how the software sees your data, is always hard. Which is why during custom report development I always also show (Select … in Manager terminology) extra information such as:
I was able to get the report to just show Credits or Debits using the Where clause, but I do need to get a better handle on how/why Manager does things so I’ll check out the post linked.