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I am sorry to hear about this situation. This is not an uncommon occurrence.
Someone in your situation can file (or threaten to file) a partition. A partition on a property is a lawsuit. The lawsuit requests that the interests of two or more individuals who share interest on the same title are severed. The Court may decide to give the property to have one of you buy out the other, or, order it sold all-together and distribute the proceeds based on percentage of ownership.
This can be costly, so perhaps the threat of this may have him agree to something fair to avoid having to go to Court which will likely simply force the same outcome, anyhow.
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