That's a bit... confusing.
I think you misunderstood the challenge. First point, the probe has to be sent full, you can't use the fuel from the probe. Then second point, you also have to show a screenshot of your probe when it crosses the orbit of Mars, and its speed must be at least of 7250 m/s. That's how we are sure that the probe is on an escape trajectory.
I suggest that you have a look at the tutorial I made to fully understand what you are supposed to do (there's a demonstration video in it).