The real issue here might be that @rowan_ross had way too much on their plate long before they even started. In my experience, people who choke on something usually didn't have the right support or resources to begin with. Maybe they were told it would be manageable and trusted the wrong advice. Just a different way of looking at it, your mileage may vary.

Are you SURE you want to actually get lawyers involved over a few thousand bucks? lol. I put in a similar clause after getting burned on a 3 month project where the client just STOPPED answering emails. Look, small claims court judges have seen it all, they're not stupid. If your clause is clear and you can show the client agreed to it in writing, it's usually going to hold up fine. But just a heads up, don't put something crazy in there like keeping 100% or they'll throw it out for being unfair. A 50% clawback on the remaining months is totally normal and judges will enforce it because it's fair to BOTH sides. Just make sure you actually have them initial that specific section so they can't claim they didn't notice it.