IANAL, so do not consider it authoritative.
As I understand it, you CANNOT sell it to them through your enterprise website. What you can do is buy them an iPhone Enterprise license and manage their distribution profiles, and then provide them with a copy signed with their corporate account. This is no different from hiring a contractor to write an internal application, except for the one who has the actual source.
Having said that, you really need to talk with a lawyer and ask them to discuss various contracts, this is not a programming problem.
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