Fund Management Related Query

Under Fund Management scenario -

After creation of PR :10 units @ Rs100 - Rs 1000 budget deducted

After PO : 10 units @ Rs 150 (includes Rs 50 for tax) - Rs 1500 i.e. 500 budget deducted in addition to above

At the time of GRN i.e. MIGO for 2 units : Budget adds upto Rs 1200 (for 8 units not received) i.e. overall budget deduction till GRN with 2 units = Rs 300

But the system should make the provision for the units for which PO has been raised but MIGO not yet passed. How to derive this scenario?