Discount in tax invoice

Question isn't very clear.

discount equal to  the amount of VAT is common practice. this discount is calculated on the net bill amount after including VAT

1)whether discount given in vat invoice is trade or cash discount?

2) how to calculate cash discount in vat ? i.e. whether before calculation of vat or after calculation of vat

3) efect of susequent reduction in invoice amount in vat