445.2. When a player is instructed to Pay a resource, that player may remove that resource from their Rune Pool if it exists there. If they choose not to, the instruction is negated.
445.2.a. If the instruction to Pay resources is part of paying costs for a card or ability, choosing not to pay those resources will cause the act of playing the card or ability to be undone. See rule 358. Check Legality for more information.
445.2.b. If the instruction to Pay resources is part of paying any other cost, choosing not to pay those resources will prevent the execution of the associated effect. See rule 201. Costs for more information.
445.2.c. Players may activate abilities that Add resources with the Reaction keyword at any time that they are instructed to Pay resources. Those abilities finalize and resolve immediately, ignoring normal restrictions. See rule 430. Add for more information.