In the State of Minnesota there is a doctrine known as the Statute of Frauds. It requires that some contracts must be in writing to be valid. Contracts involving the sale of real estate, contracts concerning the sale of goods worth more than $500, contracts that cannot be performed within one year, contracts to pay off someone else's debts, leases for more than one year, and contracts concerning a marriage must be in writing.
If your verbal agreement was that he could pay you off over more than a year then, arguably the Statute of Frauds would apply to your agreement and make it non-enforceable. However, a common defense to that argument is that of partial performance. You performed your end of the agreement and the court will adjudge that the other party must abide by his agreement.
What you will need to do now is to sue him in court for breach of contract. You clearly have a meritorious case. When you get your judgment you will have to execute on his property, bank acounts and wages.
I urge you to retain an attorney to help you locally. You cn likely find an attorney to represent you on a contingency basis--meaning you only pay them a fee if thye can collect for you, and then you pay a percentage of what they collect.
I wish you luck in collecting what is clearly owed to you.
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