<p>OP, this is exactly what department chairs are for as far as undergrads are concerned. Send the math department chair an e-mail explaining the situation, or go to the chair’s office and speak to her/him directly if s/he has office hours this late. One of the chair’s main responsibilities is to settle disputes of this nature, and, in my experience, s/he will almost always sit down with both you and your professor to get both sides of the story and develop a compromise that works for everyone. Definitely do this as soon as possible.</p>