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I'm sorry to hear about your situation. The only way to enforce an agreement where he would not pay voluntarily is to take it to court. Now luckily this is under the small claims limit, so you could bring the case in small claims court. But prior to that, Send a demand letter demanding payment within 30 days, otherwise you will pursue legal action against him, seeking that amount plus any additional damages as allowed by law. Send this letter certified, return receipt requested, as well as a copy sent regular mail. Keep a copy for yourself, as well as the return receipt number so that you can show the court that you made a demand for the repayment.
If he still doesn't pay, Do a search on the web for your county and "small claims court." You should find either a website or phone number to the small claims clerk. Ask them what you need to do to bring such a lawsuit. The small claims clerk will give you guidance on how to file this suit and how to get the other party served with notice. You will receive a hearing date, at which you should present your evidence and ask for a judgment for the amount that you should be paid back.
Hope that clears things up a bit. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for the time and effort that I spent on this answer unless and until you rate it positively (3 or more stars). Thank you, ***** ***** luck to you!