Issue in Pricing......

dear Experts,

While doing Sale Order I have come across the issue in Pricing i.e
Sales tax calculated on Base price, I gave the customer discount 50%, then net value is decreased but sales tax could not be changed, I need to change the sale tax according to base price.
Ex : - Pr00 : 20000
Net Value : 20000
JIVC : 5% : 1000

k007 : 50%
Net Value : 10000
JIVC : 5% : 1000 ( remaince same) I need to change.
Please suggest me.

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Regards,

Siva Krishna B.