I rode a 24" Holdsworth Mistral for decades even though it probably was technically too big for me. That's what the bike shop fitted me on and it worked and I was comfortable on it. My Trek 720 that I bought a few years ago in a bike swap for $120 (a good deal I couldn't pass up) is a 22.5" frame, and yes, it fits me better than the 24" Holdsworth, but the "too big" Holdsworth did give me years of good service.

So give it a go! I bet you'll be able to make the 620 work well.

PS You got a great deal!