For example, Lance Armstrong and Marco Pantani on the famous Alpe d'Huez in the Tour de France: Lance offered him the stage win if he would work with him so he could make time on his rivals for the overall 23 day race general classification:
http://www.cyclingnews.com/features/armstrong-if-i-was-the-c...
In top level professional riding, if you renege on a deal like this, you will have a seriously hard time making another deal like this in the future and that will seriously impair your professional prospects, because these deals are everywhere because of the multi-goaled nature (stage vs. overall vs. mountain points vs. sprint points) of the sport.