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 429. Add for more information.
§ 444.2.c Core Rules PDF (officieel)
§ 444 Pay
§ 444.2 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 ignored. Example: When Diana, Lunari’s triggered ability resolves, if its controller has [1] Energy in their rune pool, they choose w…
