Procuring cause when lease leads to sale

This seems to me the seller's problem, not yours, no?

From your perspective, the ideal would be for your attorney to throw in an indemnity against broker fees in the sale contract, making it 100% clear that the seller would be responsible if there is a claim.

I don't know why owners would give this free provision to a broker. The broker is getting a rental fee. Why then give the broker an opportunity for a second fee without having to earn it?

It is the seller's problem technically, but if the seller is not paying a fee the buyer can come in with a lower offer.

As for the lease issue I would say later leases with no procurement clause override original lease, but I'd need to see the original language to be sure.

it may not be the lease that is relevant, rather the listing agreement that the landlord signed with his/her agent. Ask them if they still have a copy and what it states, usually there is a clause which indicates that if the tenant buys the apt at a later date a commission is owed. This happened recently with a broker from my office they rented a townhouse to a renter four years ago, that renter then bought from the owners, and he was paid commission by the owners.

Find another broker who will do the transaction for a nominal amount - seems like that would be proof of who the procuring broker was.