Tracing User Exits for Transaction ME22N

I have a problem with the transaction ME22N. When I try to add an additional item for a certain PO, I can't save the changes because there is an error message saying one of the previously created item has to be checked under the 'Returns' field. The problem is this specific item has been marked for deletion or has been deleted. So technically SAP should not check this item before saving changes. So I think there is a customised user exit somewhere that is checking items that have ben deleted. So now I have to trace out this user exit and fix it. I have no idea how to do it. I tried to run a program which lists out user exits based on transaction codes from this link : http://www.sapgenie.com/abap/code/abap26.htm

But through the code only standard SAP user exits are displayed. I don't know how to solve this. Can anyone help me?