George Washington owned several slaves outright, but most belonged to the Custiss estate. He legally wasn't allowed to free them. We can speculate that Washington could've gotten away with freeing the slaves owned by his first wife's husband as he was very well respected and admired. But he was very concerned with precedent. So we can also speculate that. But it is only speculation.

But Martha freed many of them after her husband's death because of a series of fires that seemed to be intentional. The slaves would be freed upon her death, but she was aware that there were several people who were waiting for her to die, so she sped up the process.

Source: http://www.mountvernon.org/george-washington/martha-washington/martha-washington-slavery/