Toussaint was a pragmatist. He wasn't willing to restore slavery, but he also had no interest in exporting the Revolution to the rest of the Caribbean. Haiti is still recovering from a decade of war, and trying to simultaneously restore its economy and rebuild a stable government. He was quietly trading with the British and the Americans (under the Adams administration) even while they were fighting the French elsewhere. Unlike Napoleon, he had no interest in conquest for the sake of conquest; he just wanted a decent-sized territory to rule over. If France tells him to stir up rebellions elsewhere, he may allow some of his less trustworthy followers to try, but he's likely to tip off the British that they are coming (he has been suspected of having done this OTL). If France orders him to send his army over the sea (with what navy?) to invade a nearby island, things get dicier, but he can likely stall long enough for the moment to pass; as noted, he has no navy capable of a serious invasion, and the Royal Navy would certainly intervene.
So, what does he offer?
1) Not having to get involved in a massive quagmire; yes, 20,000 soldiers is a small army by the standards of the invasion of Russia, but it's still a significant body of experienced troops and officers who won't be thrown away needlessly.
2) Not breaking the Treaty of Amiens quite so soon. Avoiding a war with Britain (and its vast coffers willing to hire other nations to fight the French) is basically the only way that Napoleon can realistically win in the long term. I don't think that he can do it (he IS Napoleon, after all), but not giving the British an excuse would help. Especially since once the war breaks out, that army in Haiti is cut off from the metropole in any event.
3) A safe base after the wars are over. Toussaint wasn't going to invade other islands, but he was more than willing to fight anyone who invaded Haiti, and he had already repelled several foreign expeditions already. Toussaint was trying to restore the sugar plantations (and using forced labor to do so, in many cases). It might not be quite so profitable a colony as it had been, but it would still be a significant asset to retain.