Don't buy the horse until you see it sound. Tell the owner you'd like to wait and see how the injury cures before committing yourself. If he/she isn't trying to pull a fast one on you, he'll respect your decision. Do you think the horse's injury warrants the degree of lameness you've seen? You don't say how deep the cut is, but if there's no blood or infection it sounds to be healing ok.
I bought a lame horse, and I wouldn't do it again! I don't really regret my acquisition because 1. she is gorgeous, 2. I'm an eternal optimist and I think she can return to soundness although it's taking time, and 3. I hate to think what future might have awaited her in that dealer's yard if I hadn't bought her. But it's a constant source of at best, anxiety, and at worst, big vet bills. Don't risk it if you want a horse you can ride.