After blowing up the airlock to slow down sufficiently to rescue Mark at the end of The Martian, how did the crew make it back to Earth with the change in velocity having exhausted all their fuel?
The Hermes is on a slingshot around Mars, it doesn't enter an orbit. They are arranging for the MAV to launch to meet its speed as it makes its one fast pass around the planet.
After they maneuver to intercept Mark the velocity (between the Hermes and Mark) before they blow the airlock was around 40 m/s (130 ft/s).
Beck says that he can catch and hold Mark at any speed up to 10 m/s (33 ft/s) obviously the slower the better.
The difference between 10 and 40 meters per second is huge if you are having to catch someone with your arms, it is absolutely minuscule when it comes to the velocity they are actually moving at around Mars and the velocity that they reach when traveling between Mars and Earth.
The fuel reserves that Martinez talks about is the fuel for the maneuvering jets, not the main 'engine' of the Hermes - which is some sort of ion engine that has a very low thrust but can run continuously for months or years.
So Martinez has used up a lot of the reserve for the maneuvering jets getting their path around Mars such that they intercept Mark, but they still have plenty of fuel to get home.
They blew the airlock because they needed a significant change in intercept velocity in the few minutes they had available. The main drive of the ship is too low a thrust to achieve that in the same timescale. They are still travelling at a huge speed around Mars on a slingshot back to Earth.