What if you're an apple vendor and someone gives you $5 bucks and an empty bucket, with the expectation that you're going to fill that bucket full of apples and give it back to them?
In the moment where you have an empty bucket and a debt of apples, I'd call that a bucket of negative apples.
It's hilarious watching people struggle with this. The whole point is that you have to think about it - it depends on philosophy.
In other words, it's all in your head. A real, physical, "negative apple" does not exist in this world. Only the human philosophical concept of "owing someone apples" or similar such things. That's the guys' point. It doesn't matter whether you can bring representations of negative apples because those all depend on a philosophical bedrock.
In the moment where you have an empty bucket and a debt of apples, I'd call that a bucket of negative apples.