Prepayment Tax on Prepayment Voucher Entry

 Hi, When I enter a prepayment with Tax through Prepayment Voucher Entry form and I post the positive line, I get:
Account            D        C
VAT                  20
Trade Account            120-
VAT                            20-
Prep.Expense   120

And I was expected to get (as a regular voucher):

Account            D        C
Trade Account             120-
VAT                   20
Prep.Expense   100

Note:for the negative pay item, I want to use the same tax rate and tax explanation code.

I want to know, if I  done something wrong or if there is a setting that I missed.
Thank you in advance!

Best Regards,
Paula