My question involves a security deposit in the State of: Florida

I received a 30-day written notice from my roommates (who are not on the lease and would be considered month-to-month) saying that they are leaving at the end of the month and that their security deposit can be used for next month's rent because they are leaving one month earlier than agreed upon. We have it in writing that they forfeit the deposit, but have threatened afterwords that they might demand it back. Since it's in writing, can they legally take back what they said and get it back?