Not able to override tax on Purchase Order

We facing the tax issue at PO level

Not able to override tax rate at tax classification on PO.

Ex. we have setup default  tax rate 5% at OU level and the same tax rate defaulting on PO but we are not able to override tax rate from 5% to zero % manual on PO.

Please let me know, is there any setup missed.

Thanks,

Mallik