Hi Pranjal,
I think its a budget exceed problem. i have tested in my system in Txn IW32 at the time of order saving system throw the error (Message no. IW172) as you earlier said then chek in GoTO-> Logs->Cost Determination-> Message no. BP601.
If the error is same as i mention then contact to your budget manager for incresing the budget value.
regards
Rajiv Sharma