The Empire hires Bounty Hunters to track Luke (and the Rebel band), of which Han is an accessory to: if not a rebel himself.
The Rebel band (Han, Chewie, Droids and Leia) seek refuge at Bespin, as its supposed neutrality is supposed to offer them some form of protection (or at least not immediately reporting their presence to the Empire), not to mention Han's relationship with Lando.
This plan would have worked, if Boba Fett had not tracked the Millennium Falcon to Bespin, by hiding Slave 1 amongst the debris/refuge field discharged by the Super Star-Destroyer.
By plotting their trajectory (and applying some deduction) Boba would have been able to both trail the Falcon and report their destination to the Imperial Fleet.
As Solo was meaningless to the Empire (except to demoralize them by witnessing him being frozen in Carbonite, and to test whether a Human would survive the process so as to transport an unwilling Skywalker), part of his fee was implied to be Solo: Vader's foreknowledge of Jabba in the "he's no good to me dead" conversation infers a previous agreement/discussion between Fett and Vader.
So Boba was able to claim one bounty whilst picking up the necessary cargo to claim another: it's obvious negotiation.
However, there is another glaring plot-hole hidden within this sequence that goes un-addressed: maybe someone else would like to ask it (it might already have been asked!), but...
How did the Empire beat the Falcon to Bespin, if the Falcon is the
'Fastest Ship in the Galaxy' and Vader "arrived just before you did