How to control budget at Sales Order Level.

Dear All,

I want to control budget for promotion (rebate, free goods,...) at Sales Order level.

I tried with internal order. I set budget for Internal Order and assigned that order to Sales Order line item. System just control budget at post good issue (free goods) or billing (rebate), but it's too late and inconvenient.

Please tell me how to set budget at Sales Order level

Thanks

QDao