User exit for Delivery Module

Hi ,

I had a include program in Development for Delivery module, "MV50AFZ1" in program "SAPMV50A", which is supposed to be a user exit. So I transported that to production on clients requirement, but then its creating some problems for client, so he wants his previous program to be back. But I can't change this "MV50AFZ1" program in development nor in Production. So how could I bring that previous code in production.

Plz help, and if possible tell me how can I change "MV50AFZ1" program code.

Thanks.

Shailesh.