Do they need to be linked to the high concept? If this is only a temporary gig, as you say, it might work just putting it as one of the additional aspects. I think the question is kind of, what do you want to convey? Is he a runemage who is temporarily working for Freya? In that case, I don't think the high concept needs to change, just take any of the other aspects, even the trouble, if she's getting him into trouble all the time.
If the link to Freya is deeper, maybe the runemage thing is sort of secondary and his debt to Freya is more important. In that case, something like "Freya's enforcer" or something along those lines might work, and the runemage thing is just part of his other aspects. It's still what he can do, but it's more a means to an end now, instead of what he is.
But it seems like forcing both into one aspect is going to be more trouble than it's worth.