Adjust amounts for extended withholding tax

Question

Dear All,
Ii have entered invoice for a vendor A. for amount 100 with extended withholding code for 10% tax calculation. That mean system will automatically deduct tax at the time of payment. The entry will be
Exp            100
  Vendor        100 with Withholding codes
this entry is done for recognition of Expense
Payment entry
Vendor      100
   Bank          90
   withholding  10
The problem i faced is when i post another invoice for the same vendor/amount and manually change the withholding tax amount at document level.
Example
First invoice  100 withholding tax amount 10 automatically system calculates
Second invoice 100 withholding tax amount 8 manually changed tax amount at document level
Third invoice 100 withholding tax amount 7  manually changed tax amount at document level
when i manually paid the first invoice system calculate tax for amount 10
when i manually paid the Second invoice system calculate tax for amount 8
when i manually paid the Third invoice system calculate tax for amount 15. Here system added the difference which were manually adjusted by user at document level.
Is there any way that system do not adjust the previous amounts????? At the time of processing third invoice payment.

Answer 1

Dear,
if You put manually 7 into the WT amount for the third invoice, the Standard System shouln't overwrite such an amount during the payment posting. So I don't understand how the System could calculate 15. Did You change something in the Standard?
Mauri