7-9 on Aid & Interfere

This just stroke me. If you roll a 7-9, you always expose yourself to cost, danger or retribution, or you do it only if the roll you're aiding/interfering gives a miss or weak hit?

On a 10+, there is no consequence so I don't think you'd get one for Aiding. On a 7-9 or 6-, there's a consequence, so if you Aided you are exposed to it.

If they Aided and pushed the ally into 10+ territory, it should be all good for the players, I think.

On a 7-9 its a soft danger/retribution/cost. Tell them the fictional consequences and ask 'What do you do?'

It sort of becomes an additional problem or obstacle for the players to narratively deal with in the story.  It may require a move all its own or merge with the circumstances surrounding the initial Aid / Interfere impetus. This is a great example of moves snowballing!

I've had a GM run it that on a success (even some weak hits) there was no problem manifesting, and therefore the Aid-er was fine. This might work sometimes (fiction first!) but I think it's more interesting to always throw in something. "Cost" is probably the best catch-all; time, energy, positioning, the opinions of others/social harmony ("You were awfully quick to help Rothgar with that gate!") don't need to be harsh enough to be more than flavor, while still keeping the Aid move risky enough that it's not an easy call and interesting no matter what's rolled.