Exclude Planned Costs - Define Tolerance Limit - Budget Control

Hi,

We are trying to customize our system in order to exclude the planned costs of the project of the budget availability control.

Example: If once I have budgeted and released the project I add a component to the project I do not want to any error message to appear. Error should only appear once I generated the Purchase Requisition for an amount superior to the WBSE budget.

We have been trying with the customizing point "Define Tolerance Limit", trying with different combination of the Activities Groups but we haven't succeed.

Has anybody tried this before? Could anybody give me a hint?

Thanks a lot