In the epilogue to Shadows of the Empire, it is established that Luke has no intention of letting Jabba keep the droids. The epilogue actually has Luke's inner monologue while recording the message. While Luke does believe that things would be much easier if Jabba agreed to bargain with Luke and/ or the Alliance for the release of Han Solo, he does not expect Jabba to do so, and is simply baiting a trap.
It is further established, in The Paradise Snare and The Hutt Gambit, the first two books of The Han Solo Trilogy, that it is common practice among the Hutts to send a 'business' (ie; criminal) rival a message that simultaneously contains a threat and a gift. This is a part of Hutt culture that Luke, a native of Tatooine, was likely familiar with. If Luke wasn't familiar with such Hutt cultural nuances, Chewbacca certainly was. By offering Jabba the droids as a gift, he is simultaneously increasing the odds that Jabba will actually bargain for Solo's life by being friendly, showing his own cultural worth through his familiarity with Hutt culture, threatening Jabba with the might of the Rebel Alliance (Last Man Standing, a fantastic short-story about Boba Fett in Tales of the Bouty Hunters, establishes that most of Jabba's subordinates were planning to flee after Jabba executed Solo, Chewbacca, and Skywalker, as they thought the Rebel Alliance would launch a retaliatory strike that Jabba, for all his might, had no chance of surviving), and baiting his trap by getting Artoo, with Luke's lightsaber, through Jabba's security. Growing up on Tatooine, the idea that Jabba would take him outside the palace for the execution is also something Luke would have known.
As for what Luke would do if Jabba surprised him by actually agreeing to negotiate; that's a very interesting question. Luke obviously believed he could get Jabba to give up Solo by using his Jedi mind tricks, with Leia, Lando, and the droids simply as a back-up in case that failed. He would presumably have also taken the droids with him if that worked. If Jabba agreed to negotiate and mind-tricks were not necessary or effective, Luke likely planned to also negotiate for the droids. It is unclear exactly how much authority Luke had from the Rebel Alliance, but Leia certainly had the authority to make a devil's bargain with Jabba, giving him a great deal of autonomy in Hutt Space and possibly even military backing for a coup on the Hutt homeworld of Nal Hutta; far more important than controlling Tatooine and Nar Shaddaa. She could even have offered Alliance help in eliminating what was left of Black Sun for Jabba; he would have liked that.
Failing all of this, it is never established, either in the films or anywhere in the EU,what exactly Luke would have done about the droids if Jabba had agreed to give up Solo. He definitely would not have willingly let them stay with Jabba; in Heir to the Empire, he has to fight back a very angry response when Talon Karrde suggests selling Artoo to the Empire and letting Luke return to the New Republic.
It is possible that Luke, despite his recent Jedi training, was still not quite over the impulsive behaviour of his youth, and didn't actually think through all the possible permutations of his plan before launching it. After all, he didn't seem to consider the possibility that Leia would get raped while in Jabba's palace, which is a real possibility in Last Man Standing, where she is given to Boba Fett for the night as "a gift from Jabba." If Fett weren't a stand-up guy for a criminal - he gives Leia the bed and sleeps sitting up in a chair, telling her that "sex between those who are unmarried is immoral." "So's rape." - things could have gone very wrong there.
