He's (one of) the incentive man. Obviously Fett works it out to get money from both Vader and Jabba... but Jabba had had a price out on Solo for sometime. You can't say that Fett would have necessarily been willing to catch him if it weren't for the bounty Jabba had out there. That is to say, you can't say that Jabba had nothing to do it.

In fact, Jabba and Fett must be given credit for keeping Solo alive. If Fett hadn't wanted to collect the bounty that Jabba had out on him, I doubt Vader would had let him live. After what he did to his daughter...