Throughout the show, there has been numerous "deals" or "promises" for the characters to act a certain way, do a certain mission, etc. In Sundown, we see the two types of deals that Jacob and MIB have played in the past.
Jacob gave Dogen a "deal" to save his son whom he killed in a traffic accident: he would save his son in exchange for Dogen's eternal service as temple master.
Flocke/MIB offered Sayid a "deal," too. He asked Sayid to enter the temple and deliver his message in exchange for granting him anything that he wanted (a genie's wish). It is similar to Ben's comments about a "magic box" granting anything one desired to Locke when suddenly Locke's father, Cooper, showed up on the island (proclaiming to them that he was dead and this was hell).
In the theory that LOST will turn into a battle for Heaven pitting Satan's army against God's angels (with the Island being the nexus point between the two realms), if we look at the standard operating procedures of Satan and angels we find the following:
1. Satan was a master of temptation and bargains. He built his followers by corrupting them with sins by enticing them into actions with adverse consequences. Example, Satan as a snake prodding Eve to eat the apple (of knowledge) from the forbidden tree, and thereby getting both Adam and Eve banished from the Garden of Eden. They became sinners and their offspring (mankind) subject to the corruption of Satan. So goes the saying "making a deal with the devil" in order to some mortal temptation fulfilled while damning one's soul to Satan as part of the bargain. The human soul does not achieve freedom or paradise in the end. In some respects, he is worse off than not taking the deal.
2. God builds his followers through the promise of paradise through the process of redemption through faith. If you follow his rules, you will be safe. A person has free will to choose one's path, good or bad. And even if you were bad, there are means to confess one's sins prior to the moment of final judgment. In the end, one is protected against the forces of darkness.
And the debate lies in whether Jacob is representing the forces of good and MIB the forces of evil or vice versa. In the Dogen example, Jacob's bargain seems to be selling Dogen's soul and servitude to the devil. In Flocke's message to the Others, if you follow me you will be saved, may be considered a (wrathful god) calling back his people to the fold.