Discount on PO item total value

Dear All,

I am having the requirement as the discount should be taken place in PO line item total value whic includes Tax also. I am not getting any discount condition types for this as all discount conditions are calculating on Gross price only.

For eg)

Qty : 11 & Base Value : 100

Gross value : 1100

General disc. : 100-

Total Value for Tax: 1000 (Actual GR/IR value)

Tax 2%: 20 (Actual Tax)

Total PO value : 1020

Disc.on Total PO value: 10

Total PO value after disc: 1010

I want GR/IR should be 1010, but Tax should not change.

What condition type i should use and how to get it?

Thanks.