This is an interesting one and I have witnessed one of these fights in court. First, it goes back to the original agreement - if it was explicitly expressed on it by either party, then the original agreement stands, unless there was a breach of the contract at any point, even if it was not noted at the time. You or them can claim a breach once in dispute for something that happened 6 months before.

Second, the client will argue that he system was done to their specifications, thus their intellectual property. Again, what did the original agreement say?

Third, they do not know what they are into during a migration to SAP, I bet chocolate and marbles you will still be attached to this client for a while - make the best of it. Implementation is sold on 'six months' that end up being closer to 12, and in some cases 18 months. The parent companies never have a clear idea of what is going on in the subcidiaries(sp?).

Keep in mind that if a non-compete and/or an NDA were not signed you can ALSO go a sell this product to competitors.... You might want to point this out to your favor if things go sour.

Ethically the data always belongs to the customer, if they just 'want their data back', extract it and give it to them (not mirroring the data structure).

Oh, they might want you to help with the migration - make sure you adjust your rates up to the SAP consultant rates if you are going to be interacting with their team or the implementation partner. This has a couple of advantages: More money, and experience with one of the big ones.

Just my thoughts....