Maybe this is called "selling exception", as described in one of RMS's
articles (entitled something like "selling exception").
Selling exception is different from dual-licensing, where (re)distributors
may choose a license as they wish. The software is still GPL-licensed.
It's is also better than exclusively distributing your software as
proprietary software. Even someone may purchase a permission (or something
like "becoming Sponsor Middle patron or higher") to use it under a non-free
license, the software itself is still free software.
