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Unless you had a "Buy-Sell Agreement" of sorts in place from before, you need to negotiate with each other and on a buy out price. If he sent you a $21,000 text message and you accepted it within a reasonable time, then you probably have an agreement. If you did not accept within reasonable time or you made a counter-offer, then you are still negotiating. Be sure to consult your own attorney to protect your legal rights.
no not unless you had an actual agreement. It sounds like you may have had a preliminary negotiation with him. If he does not want to take the money you will need to talk to a local lawyer about filing a partition action.