E1 9.1 Is it possible to change the Tax Amount in a 2 or 3 way matched voucher?

Hi --

Is it possible to change the Tax Amount on a 2 or 3-way matched voucher?  I think we may be missing some kind of setup because it seems that we should be able to change the tax amount for VAT tax.  We can type the change but it reverts back to what is calculated by E1.  We sometimes have a slight difference between what E1 calculates and what the supplier sends on the invoice.  We need our entry to match the supplier's amount.

Any advise or direction to look in would be greatly appreciated!

Thanks,

Amy