In 1995 I obtained an agreement with the department (and university) according to which, in effect, I would be on leave without pay for one term each academic year. Part of the agreement stated that this would not affect my eligibility for teaching reductions (computed on the basis of terms taught).
In 1996 I received a teaching reduction. In 1998 I did not. Since during 1997--1998 I had had two long (30 page) papers accepted by two of the best journals in my area, Duke Math. J. and Compositio Math., and I had had four thesis students the entire year, three of whom graduated in the Spring/Summer of 1998, there should have been no question that I receive a reduction. Thus, the mathematics department had reneged on part of our agreement, and I assured the chairman that if it did it again I would resign. In 1999 it did it again, and I resigned (effective December 31, 1999).
Of course, the chairman made a rather feeble attempt to persuade me to change my mind, but by then I had calculated that I had no financial incentive for not quitting, and wasn't interested.
In order to retain the rather generous benefits the University of Michigan gives its retirees, officially I retired (rather than resigned) from the University. I am now "professor emeritus of mathematics" with all the privileges of a faculty member and none of the obligations.
Thus I am completely free to do the mathematics I like best, and to climb in the mountains I enjoy most.
J.S. Milne