With the utmost respect to you, this is a dumb idea.
Running a multinational business is nothing trivial. Where will the head office be located? What is the business form? (corporation, etc). Who are the officers? Where will official business documents be physically stored?? Who can establish the business entity, and handle things like taxes, business licenses, and so forth? And most especially, who's got access to the money (banking, merchant accounts, Paypal) when it's time to get paid?
You're taking a huge risk by being the developer on such a project. The work product will be your "baby". You might have some help in designing this with your customer, but who's to guarantee that once you're done, your partner won't disappear with your work, or be selling it branded as something else? Unless you have answered all of these questions, I wouldn't expect to earn one cent if I were you. Then again, maybe you like working for free.
Just remember - once you upload your work onto a server your partner can access, you've basically given away the business in a sense.
You might have a contract, sure enough. But which jurisdiction will the contract be enforceable in - yours, or his? (Here in the US, we usually specify that a contract will be directed by the laws of a particular state, just so it's clear). If you ever need to sue your partner, you'll have to either travel to the EU or pay an attorney there. If you can't afford to do that, then you will learn real quick how useless some contracts are when you cross international borders.
I'd give this a WHOLE lot of consideration if I were you. Don't take it for granted that you've worked with this client for some time. You don't know how your potential partner is actually conducting business in the EU, because you're not there. Business partnerships are like marriages. Would you marry someone via e-mail, in another country? Think about it.