clarify: Elsewise Power

Quick question of intent:

Elsewise Power: You can give the dark power a String to use a move you don’t have, just this once. This move can come from any playbook.

I read "just this once" as meaning that each time you use the bargain, you're trading a String for a single use of the move.  The other possible reading, though, is that "just this once" is absolute, and the bargain only works once on any given move.  Which is it?

I ask the player when they take it what they think. It's happened twice now and I've gotten each answer once. I find with MH games not going excessively long in number of sessions, either option works great.

I have only ever seen it with one String for one use. So they could give the Dark Power another String for another use of that move.

I could see the other option being really interesting too though and encouraging players to get creative.

I'd intended "one String per use," but either is fine. Ask the player and go with their answer, or tell them this clarification before play starts.