How do I stop my neighbour filling my bin while still accepting his help bringing it in?

My neighbour returned my bin from the kerb and said, “I've saved you a trip.”

It had his rubbish inside and my house number on the lid. We have identical bins, and he has mixed them up twice recently. This time he explained that he had taken mine out because his was already full, then brought it back where it belonged.

That is a different kind of saving me a trip. My bin now has little room left before next week's collection. He says I hardly fill it and he thought I would prefer its spare space to be used. I had planned to clear a cupboard this weekend.

He often brings empty bins in for three houses, including mine. It is a kind routine, and I don't want him to stop doing it because I question this one part. I also don't want to praise the help so heavily that he misses the request.

How do I say “Please keep bringing it back if you want to, but don't add bags to it without asking”? I can see how he thought an almost-empty bin was wasted space; I hadn't realised my spare space had become his plan.