Please explain when and why need to do Return PO

Question

HI experts,
my doubt
1) first i created PO using Me21n
2)I never do GR (i never receive the stock from vendor)
3) by using Me22n i changed to Return PO
4) after that i do GR and the Mvmtype 161 automatically selected by the system
4)And the stock is reduced based on Return PO stock
with out doing GR how the Return PO reduce the stock
Once i changed to Return PO and i do GR how the stock is returned to vendor
please tell me abt this doubt
actually where and which case we can do Return PO
thanks
ganesh

Answer 1

> 4) after that i do GR and the Mvmtype 161 automatically selected by the system
Now Cancel the 161 Material Document / Do + 1 then Material will come into Stock.
http://help.sap.com/erp2005_ehp_03/helpdata/EN/26/ce653a5aa1ed49e10000000a114084/frameset.htm
Please Go thru the above link for Return Po Reasons.
The Return PO Process Would be :
Create a Return Purchase Order with Return Item Indicator.
Make a Goods Receipt against the Return Po of which 161 Movement will automatically takes Place.
Create a Excise Invoice for Other Movements Using J1IS & J1IV (If the Material is Excisable)
Create a Credit Memo.
Regards,
Ashok