If we go with the theory that he took the money out of the family fund to pay the ransom, and switched it for the ringer with the intention of keeping it and claiming the Dude stole it, then there's no reason he would *not* keep it when Bunny appears safe, well and unkidnapped.

Unless there was another person in charge of the family fund who would ask questions about the lack of ransom.

And given that Maud says the money was all her mother's, there is a non-zero chance that Maud is that person. And Maud could then do what she wanted with all those bones, or clams, or whatever you call them.

And maybe Maud knew Bunny wasn't kidnapped, and knew that if someone did fake the kidnapping then her father would try and embezzle the ransom, giving her the opportunity to have some money from the fund land in her lap when Bunny came back from her wild weekend in Palm Springs. And maybe she planted the fake kidnap idea with her old friend Uli.

tl:dnr - Maud did it, maybe.