My question involves landlord-tenant law in the State of: Florida

I have a rental house in Florida, and holding a signed rental agreement by a Canadian couple with a $300 deposit reserving the property for them since April 2010. As per our agreement the tenant was going to occupy the premises on Dec.1, 2010. A few days before the due date they emailed me that they can't make it until the 15-th, due to some unexpected circumstances in the family. About a week later, received an other email, stating that they possibly come on Jan.15, 2011 and if I want to advertise my place to re rent it they would understand. Since they were uncertain with their arrival, I hired a Realtor and recently the property was rented for $200 less per month and involved some extra expenses,including a real estate fee of $335. Now they are requesting their $300 deposit back stating that "I broke the lease". Although the lease agreement does not specify any cancellation policy, I have no intention returning their deposit, since they are the ones who did not fulfill the agreement and caused me loss of rent and additional expenses.
Am I right by not returning their deposit? Please advise.
Any responses would be greatly appreciated.
Thank you very much.