pre-favor

Where you do something for someone without being asked, and then try to throw it back in their face by demanding either payment or an unreasonable favor in return.

Two classic examples of shameless "pre-favoring" are:
(1) Offering to buy a girl lunch and then afterwards pressuring her to have sex with you --- or at least allow you to touch her intimately --- in return.
(2) Voluntarily performing some household chore like sweeping or snow-shovelling, and then telling the homeowner that he is now obligated to either give you cash or buy you a pack of cigarettes as payment for your labors.