How do I clearly decline a neighbour's date invitation after saying 'maybe another time'?

My neighbour asked me for coffee, and I said, “I'm busy this weekend—maybe another time.”

I meant to decline without embarrassing him. He heard a scheduling problem. Yesterday he suggested next Saturday and asked which café I preferred.

We live opposite each other, and I like our friendly hellos. I don't want to date him. He hasn't been rude or pushy; he has followed up on the opening I left. That makes the correction feel worse than saying no would have felt the first time.

I considered saying I am seeing someone, but I'm not. I don't want to keep inventing a reason that might expire or get contradicted in the lift.

How can I say plainly that I am not interested in a date while leaving room for ordinary neighbourly contact? I know I can't control whether he feels disappointed. I want to correct my answer rather than keep postponing something I don't want.