I'm considering developing a project based on a modified version of some GPLv3 code. But, I need to use some proprietary library. This is a for-profit endeavour, in which I plan to sell the modified GPL code with the GPL license to a client and charge for the unitary closed-source cost of the proprietary library, but I cannot change the terms for the proprietary library. I can get license to share the headers or equivalents to them, but surely not to the library itself.
Is that okay? I believe I'd be violating the GPL license terms, but I'm not close-sourcing anything I have access to.