Hi Alagan
Please go to the PO, tick the field free item(next to net price)
Now, you can keep the netprice blank and save. By default, the invoice checkbox in invoice tab wil be disabled. In case you find that invoice check box is still ticked or is throwing error due to it, you have to untick it. in case it is mandatory field, make it optional in the PO layout config (explained in one of solutions above)
If you are not ticking free goods and directly setting price as zero, this will work only in versions pre EHP5 ECC6
Regards
Vivek Singh