A guide on how to ask for a refund and get your money back, including sample request for refund letters. A security deposit return letter serves as formal documentation that details the return of the security deposit to the resident after they move out.The letter should state whether the deposit is being returned in full, partially returned, or not returned at all. We've included a sample letter to return security deposit funds for a full refund, partial refund, and no refund. You are now over a week late returning my deposit. How do you write a letter asking for a security deposit? If you don't know it, you can find it on the Cook County Assessor's website. Fill out a complaint. In suburban Cook County, the landlord must return the security deposit within 30 days after the tenant moves out. Click here to read about Cook County security deposit law if the unit is in suburban Cook County.