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How much did you loan out? When do you expect the borrower to pay you back? If multiple payments will be made, agree on specific dates for the payments. Especially if you're lending money to a friend or family member, it may seem a little extortionate to charge interest. But there are several good reasons why you may want to charge the person you're giving money to a little bit of interest:  If you're giving away money without interest, you're losing money. You're losing purchasing power (the ability to buy and invest with the money you're lending) and inflation is outstripping your money. If you charge someone interest, the borrower may be more likely to pay you back quicker than had you not. Think about it: Interest lasts as long as the life of the loan, so if the borrower holds on to the money longer, they'll end up paying more interest. Don't charge more than 15% or 20%. In fact, interest rates above 15% or 20% might not even be allowed under predatory lending laws. So keep the interest rate at something manageable and both parties will be happy. Include your signature along with your legal name. Have them put down a signature along with a legal name. Although a witness does not make or break the IOU, it's helpful if you ever need to go to court. A witness might be able to prove a binding verbal contract took place.

Summary:
Include the date and the amount being borrowed, or the amount agreed on for the service or product. Include a due date for return of the funds. Include how much interest you will charge. Sign the document yourself. Make sure the other party signs the document. If possible, have a witness (optional).