Blindsided by supervisor

I'm in a bit of a predicament. My supervisor for over a year now has completely blind-sided me by first promising to change a grade for a course I had failed to lying about it being possible. The problem I'm having is largely due to the politics within my department. I have never failed any course in my life apart from the one mentioned, and that was due to me being late (by an hour) in handing it in due to unavoidable circumstances. He wrote an email to me the next day and said he'd accept my work. However, that never happened and now 2 semesters later I'm still trying to change it. The policy at my school allows me to change the existing grade by adding the missing coursework component to it. But, my supervisor keeps lying to me about it even though the head of administration verified it for me. He wishes to for me to retake the course in summer even though I've already passed the exam section of it. I'm not sure what to do in terms of trust issues. I have coauthored papers with my supervisor and he has always been somewhat of a mentor to me. Him blantantly lying to me has somewhat turned me off of academia.