§ 357.3 Core Rules PDF (officieel)

A player may not pay costs during this step that will deterministically result in illegal choices or actions later in this process unless they have no choice. Example: A player plays a card which reads “as an additional cost to play this, you may kill a friendly unit. Give a friendly unit +2 [M] this turn. If you paid the cost, give that unit +7 [M] this turn instead” If they chose to pay the cost, they must choose to kill a unit other than the targeted unit unless they have no choice.