I received a question in my inbox the other day for which I know not the answer to and was unable to find the answer, so I thought I would ask you guys if you knew.
“My question pertains to the sale of foreign rights and self publishing. I’ve sold rights to a foreign publisher, but how does self publishing my second book affect my prior sale and future foreign rights sales?”
Any answers for a fellow writer?
Self-Published Authors Helping Other Authors 
What does the contract with the foreign publisher say? There might be something in there about future books. I have a friend who signed up with a publisher who said that she has to submit every new book she writes to them so they get “first picks” over another publisher. I’d read the contract over, and if you have questions about, a good lawyer who specializes in contracts could be invaluable.
BTW, I’m not an expert on the subject. This is the only thing I can think of as a starting point.
The first question regarding the first sale is whether you sold these rights directly or if you had a publisher who then sold the rights arising from your sale of the rights to them. As Ruth says, it all comes down to the wording of the first contract for the first book. You need to check if there is any commitment regarding future books, and it is a good idea to familiarize yourself with all terms and conditions so you do not inadvertently breach the T&Cs.
Update: The publisher contacted the author who asked the question.
It seems that her foreign right sales are suffering because she went indie. They tought her that they were in the process of negotiating separate foreign rights contracts and that as soon as she released her second book in the series they backed out and the publisher she’d sold the rights to actually considered pulling the first book. They haven’t, but it surprised her that they would do this because she thought the stigma on self-pubbed authors had diminished.