Before hand- if you back out for any reason, then you don't have to. You would probably have to find another agency to work with if you wanted to do it again... unless your agency supported your reason for backing out. Sometimes the parents just turn crazy and you have to cut ties.
Afterward- In the contract they have designated people to take over custody if something were to happen like a plane crash and both parents die. If those people were to also back out, I'm not totally sure what would happen. I'm in no way legally responsible though.