Long story short: we provide our software as SaaS, and for our type of business (online ad delivery), due to the complexity of the applications, there's a whole industry of people who make money doing the actual operations of the application (called ad ops).
Now, our application is sold on a usage basis (use more and you pay more), and someone wants to start reselling our application by directly billing his clients instead of referring them to us.
What can go wrong? What can we forget? This seems like a no-brainer, since they can only make us more money, but I can't help but feel there's a snag to it.
What can go wrong? They can do a better job of marketing & service than you and effectively build their own brand on the back of your efforts, before dumping you for another supplier or an in-house development.
I suggest you develop a reseller product and approach it as a whole new channel. You should consider whether to use push or pull marketing or whether to require co-marketing efforts linked to VAR channel discount points. Perhaps you will have minimum volume requirements and tiered pricing, with white-label option at additional cost. You should consider exclusivity requirements - on both sides, and examine the scenarios in a competitive context. Since this is effectively a revenue-sharing partnership, you should have a standard heads of agreement and standard T&Cs ready to go. Oh, and prime your accounts people for the additional reconciliation burden.
In other words, you need a channel management strategy. All this puts you in a much stronger negotiating position and is much better than allowing yourself to be led into it blind by a savvy customer.
If none of the above is worth the investment, don't do it at all.