Dear Ajay
The point which confused me was you said 'if it does not work with real order, why don't you acquire asset with real order...".
As advised, I created a real order and budgeted. Now I posted a direct asset acquition through vendor (F-90), the system has checked the budget.
Hence, the budgeting will be OK through Real Order also.
Now I will post service e expenses also and try to settle to the real asset.
Regards