Huh. I've read in a comic (Bounty Hunter series) where Lando comes (between EP V and VI) to a small moon on a remote system to negotiate with a Hutt acquataince of him. turns out that, when he arrives there, Dengar, 4-LOM, Bossk and another Bounty hunter is awayaiting for him there, following the orders of that Hutt, that want to catch and kill him, thanks to an old game between him and Lando, where the last (probably) cheated.
Well, to make a long story short, Lando kick the Hunters butt with the help of some locals, and proceed to the Hutt's camp to finish him. When the local population are about to turn the Hutt into dust, Lando save his life with a request: That he recommend him to Jabba, as a guard corporal. The story ends with him departing in the Millenium Falcon.
A acceptable guess is that they had a plan but it turned out all wrong.
I, however, think that this was exactly their plan. Once they were all imprisioned and out of Jabba's palace to be eaten by Sarlacc, they could manage to free Han on an open environment.