The best option you have is to ask for the money back. If they don't reimburse you, it might be prudent to 'procure' items that they own of a similar value to what they owe you until such a time that they do decide to remit payment. If they choose not to, resell these items. Be aware that when you try to collect money from a 'friend' that you may in fact lose their 'friendship' (if they ever were really a friend in the first place).