Negative budget due to freight charges

Hi,

In purchase order, system is considering material price as commitment and freight charges is not considering. But when we do MIGO, system is calculating both material price and also freight charges and budget report is showing values in negative.

Ex: Budget for WBS is I00 and material price is 100 and freight charge is 10. Until we do MIGO available budget is zero and once we do MIGO available budget is showing as -10.

I think this is due to condition type setting. Can some tell me how to set a particular condition type to be considered for commitment in PO.

Thanks

US