If you choose the second option, and it is not cash, what is it? Knowledge, advertising or something else. just don't understand
Like I said in my post, you can "pay" by contributing time, expertise, or code to the library's community/ecosystem. You could make publish your app under a Free and Open Source license to benefit others. You could submit patches for new features of bug fixes to Qt itself. You could give your time to teach others how to use Qt effectively.