For ex: if tax code require AB and user puting AC then system has to give error for this input.
Normally Tax code cannot be inserted by user at Transaction level..
By the way what was the pricing type you used in billing???
Is there any difference in validity dates of tax condition values in sales order/billing??
(different tax codes based on condition record validity??)
Can you please fix your pricing in billing(if it is OK) and cal also make the fields Tax classifications in display as suggested...
Pl update..
Phanikumar