A lot depends on what you think is “reasonable.” It clearly could be reasonable to prefer UM over CMU. A number of posters think (and I agree) that it’s also reasonable to expect someone signing a binding agreement to make a reasonable attempt to determine ahead of time how likely it is they will be able to fulfill it, and then make a reasonable attempt to try to fulfill it before saying it can’t be done. It’s not clear if that’s what happened here, but we don’t know the whole story.
On the flip side, the school needs to offer a FA package with a cost that could be reasonably fulfilled by the student and parents if it wants the agreement to be held to. We don’t know if that’s the case either.
It’s not the end of the world if someone just says, “No, I can’t pay this and I’m not going.” It’s just better if people follow the system they enter in to, even if it’s one like ED, which I think has some negative effects overall.