Comment 1 for bug 1155606

i don't think a constraint would be a good idea as they may be use cases where you want a different tax amount than the debit/credit * coeff (python code, fixed amount, child on children...).

on the other hand, maybe we should restrict the tax_amount field use in the manual encoding so that the user always use the tax field instead of filling by the hand tax amount and tax code. I'm not sure, actually... but in all case, this is not a bug, more a enhancement proposal ;-)