Customer Master & Sales Order

Hi Experts ,

We have a scenario as below.

A certain Customer say XYZ is valid for 3 countries say china , japan ,Eur .In XD01 for this customer in billing part and Tax portion we have maintained Tax country as China ,japan ,Eur and Tax Classification as 0,0,0 for this customer .Some values of this Tax classification for some countries may be blank also.

Now we have a sales order where in the header section (Additional Data Tab) we have a field called "TAX".

We want to keep a condition that if the customer has China in customer master "tax country" column he should maintain "0" in Tax field in SO , other wise he wont be able to save the sales order.

How should we proceed.

Thanks in advance

MM