Project forecast problem

Dear ,

In AX Project Module 2009 I am trying to do the validation to not exceed the Budget for an investment project

the Amazing that the system is do the validation on the expanse forecast quantity and leave the cost amount

even in my case I have no issue with the quantity of the expense I just need it to concern the value

in other hand the forecast for the items is allow exceed the budget if you do your transaction from the project item journal even it stop you if you do the same from purchase order

I will appreciate any valuable input

Thanks