New Pricing procedure required in retro billing target document

Hi All

i have a requirement at client site for retro billing. I am working on the quality system for testing. i have maintained settings for pricing procedure determination with credit memo which will be created in retro billing, with reference to original invoice. But new pricing procedure is not coming in target document. It is picking pricing procedure of source document. Are there any settings required in copy control (VTFF) thru which same can be done??

Thanks

Deepak Mehmi