You should talk with an an attorney if you are unclear about the terms of your contract.
I'm not an attorney but on a standard florida contract "contingent" on the buyer obtaining financing, the buyer has a responsibility to notify you by a certain date if they are not able to obtain financing.
If, let's say your contract was to close in January and yet you're only getting notified right now that the buyer couldn't get financing, you may have a good case for retaining their deposit. This is not legal advice... I am just aware of a small claims case where an owner retained a deposit due to the buyer's negligence in reporting they couldn't qualify for financing in a timely manner.
So hire an attorney for a quick review of your contract. Depending on the amount of the deposit and the length of time you kept the house off of the market awaiting the buyer to close, it may be possible that you can keep the deposit.
Also look at your listing agreement and it may say that the listing agent gets to keep the deposit.
Hire an attorney to review your contract if your listing agreement entitles you to keep 100% of deposits and the deposit is over $500.
This is an excellent Florida firm: