Vader was the one who put Han in Carbonite, on Bespin Cloud city, and gave him to Boba Fett as part of his reward for locating him,

Boba Fett in return brought a carbonite encased Han to Jabba in return for the reward for bringing in Han for dumping Jabba's Spice along the Kessel run while being chased by Imperial starships.


Well that's my 2 cents on the matter!!