If your buyer has $ 58,000 in cash after buying your business, why don't you try to negotiate with the landlord and the buyer to tie up the cash toward securing the rent until your lease is expired? How much is the rent? How long would $58,000 last if it was used to pay the rent? Until after your lease expires?
Based on an offer like this, you may have grounds for accusing the landlord of acting unreasonably if he rejects such an offer. Similarly, if the cash was tied up to secure the rent payments, why would you be unwilling to accept the risk to guarantee the rent, with a cash guarantee like this?
As your lawyer pointed out, it doesn't seem worthwhile to go and sue the landlord for unreasonableness. Trying to reverse the positions, suppose you wrote to the landlord, outlining "an offer he can't refuse", and declaring that you will vacate the premises on some date, here is a substitute tenant, the rent is guaranteed to the expiration of your lease, there is no good reason to refuse this buyer as a substitute tenant, if he refuses him, then he is guilty of unreasonableness in violation of the lease, and inviting him to take the matter to court. Then he is the one who has to decide whether it is worthwhile to sue you over the rent for the remainder of the lease, and that there is some chance that he might lose, maybe a good chance.
This strategy has some risk, of course. I personally don't believe in bluffing, so I suggest this strategy as a committment to fight. Calculate how much rent the landlord stands to lose if you vacate and he doesn't take the replacement tenant, and then consider if it is worthwhile for the landlord to sue. Compare that with what the landlord will get in rent from the replacement tenant, especially since the rent for the remainder of the lease is guaranteed to him. How can he lose?
Remember that I don't advocate bluffing. Before beginning such a strategy, be prepared to see it all the way through.
Also, keep in good touch with the landlord, so thst once you give him notice of vacation, you can continue to negotiate with him and try to convince him that taking the replacement tenant is a wise choice.