Maintain Budget tolerance limits in SPRO path and activate availability control in KO30.
The Menu Path is SPRO -> IMG -> Controlling -> Internal Orders -> Budgeting and Availability Control -> Define Tolerance Limits for Availability Control.
http://help.sap.com/saphelp_46c/helpdata/en/55/b122ca9d4e11d189b60000e829fbbd/content.htm
Hope this helps