I am pretty sure he was accruing 3 days of obligated service for each day he spent in training at Walter Reed. He paid back some in Residence, but he had a ways to go yet.
I think that you are correct in the assumption that Hasan was obligated to serve in exchange for his training. One of his relatives mentioned that he had offered to repay the military for his education in exchange for an early out.
Refusing deployment to the Middle East makes no real sense because the man was support, he was not front lines. He was never going to be asked to take arms against fellow Muslims. His job would have been no different in the Middle East than it was at Fort Hood.