Purchase order to Asset non imposed the Internal Order.

I'm entering a purchase order of an asset tied to an internal order of investment. The order has a budget X. If I enter a purchase order with value greater than the amount budgeted in the internal order, the system allows me to create the request and not imposed the internal order. In the report S_ALR_87013019, the order has no value released.

How can the system do not allow the purchase of assets with values greater than estimated and how to make that encumbers the purchase order and appear in the report S_ALR_87013019?

This only happens in the purchases of assets!

Thanks in advanced!

GLMacedo