Ask a legal expert if you can grant a free, "source-available," and irrevocable license to them. Also, one with a patent grant. Doing this isn't the same as using MIT license for that specific project because that would let people escape your copyleft. Instead, you're keeping your code copyleft while granting a separate, proprietary license which has some benefits of open-source code.
Since people worry about abuses, I advise anyone trying to get open-source projects to take non-open code to do one extra thing. Release your behavioral specs, the API, and the data formats under an open, permissible license with patent grant. As in, they can legally and easily swap your code out for something compatible with less likelihood of breaking changes. Call it future-proofing a little.
Here's an example for clarity. Let's say your code is a parser. You define its inputs, invariants, outputs, tests, and error-handling in open specs (and/or API code) others can use. The storage of the result is JSON with a published format. Then, your actual implementation is not open. If they decide to drop it, they can use the same API's, output a result in the same format, and so on. There's less risk in a port. They might be more likely to adopt your code.