Burrenton Forge-Tender

609.7a If an effect requires a player to choose a source of damage, he or she may choose a
permanent; a spell on the stack (including a permanent spell); any object referred to by an object
on the stack, by a replacement or prevention effect that’s waiting to apply, or by a delayed
triggered ability that’s waiting to trigger (even if that object is no longer in the zone it used to be
in); or, for certain casual variant games, a face-up card in the command zone. A source doesn’t
need to be capable of dealing damage to be a legal choice. The source is chosen when the effect
is created. If the player chooses a permanent, the effect will apply to the next damage dealt by
that permanent, regardless of whether it’s combat damage or damage dealt as the result of a
spell or ability. If the player chooses a permanent spell, the effect will apply to any damage dealt
by that spell and any damage dealt by the permanent that spell becomes when it resolves.