Every manager can force 1 fight between one of his fighters and another fighter for free. After the free forced fight is used you must use gym credits to force fights . The higher rated the fighters the more credits needed to force the fight.
Fights may be forced against manually or automatically scheduled fighters as long as they aren't already scheduled and their manager has logged into the game within the last 72 hours.
A fighter cannot be forced until 24 hours after his last fight
World and regional champions can always force fights for free. This is one the perks of being the champ - hand picking opponents. If a fight is forced between two fighters and the regional title is vacant, if the timing is right the fight may be tagged as a regional title fight.