The Hack and Slash move says: “On a 7–9, you deal your damage to the enemy and the enemy makes an attack against you.” As written, that allows the GM to offer a choice like “you do your damage. The monster attacks back; you can either take the damage or be shouldered aside and the monster will have a clear shot at the wizard” . . . but not “you can do your damage, but if you do, you’ll be out of position and the monster will have a clear shot at the wizard. Do you want to?” That is, on a 7-9, the rules say the character has definitely hit the monster for full normal damage. Any hard bargain the GM can offer as part of the monster’s counterattack happens separately from that.
On the on hand, it seems like a bad idea to offer bargains as partial successes that could result in nothing happening. So having something definitely happen is a good thing. On the other hand, having characters be so likely to deal damage every time they roll makes it hard to keep monsters alive long enough to be interesting. How do you all handle this?