Difference between Print Invoice and Sales update amount

Hi,

We are in JDE 9.1.

For ex.  Qty - 1 Unit price - $ 100, Discount - 2, VAT @ 6% = 100-2 = 98 x 6/100 = 5.88 and the Invoice amount = 100 + 5.88 = 105.88.  This amount is perfectly matching when we do the Print Invoice. 

But when we do the sales update the Invoice amount changes it takes $ 100  + 6 VAT (100 *6/100) =  106 ignores the Discount of @ $ 2.  Invoice amount = $ 106.

How could this be solved. 

Thanks

Vidhya