Probably caused by utilizing the wrong method in an older version, where one had to manually enter the paid amount instead of selecting “mark invoice paid”
Step 1. Check the affected clients and make sure that the actual paid sum is correct, regardless what the “minus” amount says.
Step 2. Make a backup, or find a proper host where you can do an easy roll-back in case you mess up.
Step 3. Open the database of Invoice Ninja and find the “clients” table
Step 4. In the column “Balance” you will see a negative amount. Double-click in that field, enter 0.00 (zero-point-zero-zero) and press enter.