I put in a 30 day notice to my property management, along with dropping off the 2 apartment and mail box keys at the beginning of the month. I just received a notice in the mail from the property management stating they were withholding almost 2 weeks of pro-rated rent and charging me for keys from my security deposit. They told me I didn't turn the notice in until half way thru the month which is not true. I didn't get a copy of the notice I gave them (my fault) and they conveniently lost the one I turned in. They also stated that I never turned in the keys, when I turned them in to a lady at their main office.
Did you type up the letter? If so, don't open the file, but look at the last modified date and does it match when you gave them the letter? If so, that is one piece of evidence.
Then, does this office have video cameras? If so, they would record when you dropped of the notice and the keys. The only issue is, if this was more than a month ago, any video may already be gone. But there may be other evidence, such as where is the receipt for the new keys if they had to make them.
Otherwise, find an attorney who will take your case, as a nasty letter from an attorney outlining that the they would be liable for double the amount withheld and attorney's fees will likely get a result, whereas as letter from you likely will not. Some attorneys will take this type of case on a contingency basis, as you could recover more than what you are owed by the apartment folks.
This response is only general information and is not legal advice. It does not form an attorney-client relationship and should not be relied upon to take or refrain from taking any action. You should seek a qualified attorney before taking any action related to your inquiry.
Attorney Stevens has given you some great suggestions on how to gather evidence to support your position.
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