Should I write a Thank You Letter if I will probably refuse the job offer?

Hello. I just had an interview, and it went very well. I loved the working environment, and the hiring manager seemed to like me. The only problem is that they offered me much less that I expect as a salary. I am almost positive that I will not accept the offer if they pick me or will try to negotiate the salary. My question is, I know that it is the right thing to do to send a Thank You email after the interview, but given that I probably will not take the job, will it not be misleading to send the letter? Given that the manager already told me that it was my decision whether or not to accept the job with such a modest salary, I am afraid sending a Thank You letter may give them the idea that I am ok with the salary. Please let me know what you think! Thank you so much for your time!