It sounds as if your landlord returned your security deposit with a letter stating that landlord was keeping $50 for a pet deposit.
The landlord should have given you 15 days to object in the same notice he/she informed you that a portion of the security deposit will be retained.
However, your remedy is to take the landlord to court for the $50. If your lease contains a clause that the pet fee will be deducted from your security deposit, you should try to find a lawyer to assist you.
The foregoing are comments to a general question of law, and should in no way be interpreted as legal advice. This information does not create an attorney-client relationship. The choice of a lawyer is an important decision and should not be based solely upon advertisements.
You do not have to object. Your appropriate remedy when you disagree with what the landlord is taking out of your security deposit is to file suit against the landlord in small claims court. You do not need a lawyer to do this and your can get all of the forms you need from the clerk's office in your county courthouse.
Disclaimer: This answer is provided for informational purposes only, does not constitute legal advice, and does not create an attorney-client relationship. Actual legal advice can only be provided after completing a comprehensive consultation in which all of the relevant facts are discussed and reviewed.
I am presuming by your post that the landlord indeed sent the security deposit claim letter to you via certified mail within the 30 days you vacated and that the letter comlied with the requirements of Florida Statute 83.49(3)(a). It really isn't proper for a landlord to deduct the amount prior to your 15-day objection period. The aforementioned Statute requires the landlord to notify you of their "intent" to impose a claim; not that the landlord has already deducted and is sending you payment for the difference. If after 15 days of you receiving the claim letter from the landlord and you do not object to the deduction, the landlord gets to keep the amount and send you a check for the difference, so long as the damages claimed do not exceed your deposit. However, even if you object, the next course is to file an action in small claims for the $50. You will spend more than that in filing and service of process fees, plus your time dealing with this matter. The court has the authority to award costs and fees to the prevailing party.