Hi Sangram,
For subsequent transactions, if you flag entry FLG_SES_OVERWRITE in FM01X as active, system will be using FMDERIVE.
However you still have an issue with PO. PO with account assignment U will not create any commitments. So, you start to consume budget at the point of SES instead of PO.
Since it is a U, where did you specify your GL account?
Regards,
Ming