If money is owing, it should have been paid, but it has not been paid yet. The bill is still open. You can say there is ten pounds owing, or the amount owing, meaning the unpaid part. It is a slightly formal, money-and-paper word — shops, rent, invoices — not how friends usually talk about borrowing five dollars (they would say you still owe me). Though actually, English also uses owing to, which looks the same but means “because of”: the match was cancelled owing to rain. That owing to is a set phrase about reasons, not about unpaid cash. The sense here is only the money that has not arrived.