How to turn off Automatic Calculation for the tax rates on AP Invoice workbench?

Question here is If my tax % is 7% of Invoice Total - $100 then it will calculate - $7 but if i try to override to $6 then system is putting reversal entries automatically. Currently reversal entries are showing up only for human error. That's the reason we wanted to show 7% but the amount should not get calculated as $7 automatically.

FYI. If we end date the tax rates, then we cant re-enable it as per the tax documentation. (since its migrated data from 11i and not re-implementation)