Service order posting date error.

In DBM Service order posting date is taken from the controlling document because of that error comes as

Document date 03.12.2011(Curent date)is greater than billing date 02.12.2011(Date given in Billing date in service order header)

But this error is not coming in sales document..

I need to post service order in the date i mentioned in the service order..

How dould i resolve this issue..?