The problem with cashing in and losing is that it cheapens the integrity of the MITB match. Money In The Bank has become one of the main attractions of Wrestlemania and people generally wanna see the person who wins it, win a world title. It's similar to when someone wins the Royal Rumble, they're expected to walk into the Wrestlemania main event and walk out a champion. (this year not withstanding but Orton was a heel and fans wanted him to lose anyway). Having Punk lose will just make the briefcase less valuable and people will become less interested in the concept if they carry around this prop for months and don't utilize it.
The solution is to have Punk win it in a way that hasn't been done before. The pre-announcement thing isn't bad and it could work but a fresh idea like having a world title challenger get attacked backstage and not able to compete for the world title that now. When the champion comes out, who is his replacement opponent? Cashing in the briefcase, CM Punk. Have him win it and it sets up a problem with the ex-champ and now with the challenger who never got his shot.