It would be entirely ordinary for someone to say in English "You have a second?" in opening a conversation.
Yes, perhaps in very informal casual conversation but on a teaching sight we need to stick to the formal question format. We cannot possibly reflect every regional or nonstandard expression.
We use this expression with high frequency in the US when we need to speak with someone. It’s a rather polite way of getting someone’s attention for a short period of time. Do native Greek speakers use it with the same intention and just as naturally?