I don't think Obama is painted into a corner on this one because he gave himself a classic 'out' clause in his commitment. Bill Burton said that they would “aggressively pursue an agreement”. He only committed to pursue an agreement, not make an agreement.

There are plenty of preconditions that Obama could stipulate that would make it nearly impossible for an agreement to be made.

For Example: There is the issue of third party spending: would they have to pledge to denounce the third party spending? Can they actually stop free speech for the sake of their agreement? Unlikely.

Also Obama could require that once McCain gets 1,191 delegates that he stop spending any money and go straight to the General election fund because his primaries are over. One could argue that McCain is already in the general election and all his current spending should count toward his 85 million.

In sum all Obama has to do is say that a fair agreement could not be made and then they can start blaming each other, but that is better then breaking a commitment.

Campaign finance rules are particularly difficult to dissect, but that makes them nearly useless for issue politics. Most people don't understand campaign finance and will therefore not care.