To send gold to another player, you have to buy something from them. Most commonly, when you want to just send gold, the person to receive the gold sends you a CoD (cash on delivery). They do this by selecting a worthless item, then sending it to you as a trade that has a price of the agreed upon gold attached. You accept the item, and the price, and the gold gets transferred from you to them.

Not sure how clear that was, but the long and short is that to send gold to someone, they have to initiate the exchange, not you. So you will have to contact them and ask them to CoD you for the refund.