So let me see if I have this right... B made a cash offer & signed a contract. B's agent asked you to let B out of the sale for "medical reasons". However, B refuses to give you a written termination of contract. It sounds to me like B wants to complete the sale & B's realtor & MD are the ones who don't want B to fulfill the contract. The only question is whether B is competant. The fact that he's 92 is not absolute proof of incompetancy -- I regularly get my butt kicked by 92-year-old bridge players!
Until & unless you get a termination of contract signed by B, OR you get a copy of a court order holding that B is incompetant to manage his own affairs & was incompetant when he signed the contract, YOU ARE BOUND BY THE CONTRACT. You can go through some rigamarole regarding anticipatory breach & assurances of performance, but it's not worth you time. Closing's set for Friday! Just send B notice of where & when closing will occur and show up with the papers.
If B doesn't close, send B a written notice that he is in breach & you have put the house back on the market & are keeping his earnest money as liquidated damages. If he still wnats the house, he has to close before you accept another offer.
If you have damages in excess of the liquidated damages amount, you may be able to sue B, depending on the contract terms. This would kick in if you can't sell the house for as much $$ to the next buyer, or if you have to make some repairs before selling. If you sue B, B will likely defend by claiming that he was incompetant & can't be held to the contract. Furthermore, since the contract was void ab initio, you can't even keep the earnest money! In fact, if you can't sell the house for what B offered to pay, that would be evidence that B IS incompetant, since a competant person wouldn't have paid that much or would have required certain repairs first.
All in all, I think your best course fo action is to keep the earnest money if B doesn't close on Friday & just put the house back on the market. This is why sellers who receive identical offers often accept the one with the higher earnest money offer.
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This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.