A verbal agreement can be enforced but proving what the agreement was is the big problem which is why a written contract is preferred. Some oral contracts cannot be enforced because of the Statute of Frauds. Because you list no details, it is not possible to determine if the oral agreement you are asking about is or is not barred by the Statute of Frauds.
You may consult a lawyer in your locale (I am in Chicago, not FL).
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Too many variable exist to predict the outcome. Good luck to you.
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Oral contracts are just as valid as written contracts but they are more difficult to prove. The statute of frauds, mentioned by my colleague, requires a writing in certain kinds of transactions: real estate is one, and a sale of goods in excess of $500 is another.
Yes... But is the contract for something that is legal? Has the person partially performed on the contract? Do you have any witnesses. If you sue you will have the burden of proof so you must prove more than 50% that the other person is wrong. If both of you are equally believable the judge must rule for the other party as you have not meet your burden of proof. As a practical matter - is the other party worth suing? do they have money even if you win? Good luck and I hope this helps!