Hi,
As you already paid to vendor, no need to cancel invoice with quantity 1 and value 3600.
Better cancel goods receipt document for your PO by activating in SPRO
SPRO-MM-IM- GR-For GR based IV, Reversal of GR in spite of invoice ( select movement type, Exa-102)
Once the above SPRO activities done , follow the steps
1. Cancel goods receipt document for your PO
2. Correct PO quantity 1 and value 3600
3.Again carry out goods receipt for your PO with quantity 1 and value 3600
Regards,
Biju K