bait and switch

An unscrupulous and sometimes illegal sales technique, in which an inexpensive product is advertised to attract prospective customers who are then told by sales personnel that the inexpensive product is unavailable or of poor quality and are instead urged to buy a more expensive product.

(by extension) Any similar deceptive behavior, especially in politics and romantic relationships.

To lure someone with an offer of some kind, but substitute something of greater advantage to the offerer.