Tax error in Purchase order

Hi,

We are not able to change the TAX of purchase order .

Navigation: Query Purchase Order-->Actions-->Manage TAX-->TAX page--> Additional Tax Information, update the tax with Exempt TAX (VAT 0%)

When doing this action and saving the changes we are unable to change the TAX to 0% and this still remains in TAX 5%.

Please provide suggestions for solving this issue.

Thanks in Advance,

Vinay.P