Hi,
I'd ask what needs to be recorded in this scenario and does the change impact anything else after the opportunity is won? The system allows opportunities to be re-opened, edited and won again so the user can easily update the opportunity value.
You could consider adding negative opportunity product lines, that way the value of the opportunity will be correctly decremented and the value of rebates could also be charted / monitored.
You don't mention what happens in your system after the opportunity is won but if you are using quotes/orders etc then their values will also need to be considered when applying the rebate.
Rob
MCTS. GAP Consulting Ltd. Microsoft Community Contributor Award 2011 & 2013