I'm sorry to hear you're having such challenges. Unfortunately, there is no good answer for you other than saying you should speak to a family lawyer.
That being said, your question of "Can I sue" is really "Should I sue". Anyone can sue anyone, but that doesn't mean there is actually a case or that the claim won't be dismissed.
Suing your soon-to-be-ex for $500.00 and "dragging your name through the mud" is not really a claim, particularly when you agreed to be a co-signer/co-debtor on whatever it is that he needed to pay.
Reach out and try to work this out with him. You may need a family lawyer for the bigger issue of the pending divorce, and perhaps the $500 could be addressed in that forum.
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File a an action for dissolution of marriage. Technically these bills are marital in nature and are divided 50-50. Allege in your Petition that the Court should depart from an equal split and consider a disparate distribution since your husband ran up these bills in your name wihtout your permission and after the separation. You must swear to the facts in the Petiiton . It would be a good idea to have a lawyer help you. If you are not going to do that at least read Florida Statute 61.075. You can find it on the Florida Senate website. Good Luck.
This answer was provided as a courtesy to you and no attempt was made to establish any type of attorney/client relationship.
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