My husband and I entered a business contract with a partner, whereby we owned 50%. with it going to the surviving spouse. My husband passed away over a year ago and I received a note from the partner saying I would no longer receive anything because he had a gentlemen's agreement with my late husband. I knew nothing about this and my husband would never had agreed. Both our names were on the original contract and I used my 401k to pay for the business.
If there was any oral agreement, it may or may not be binding, depending on the state. I vaguely recall a situation dealing with oral contracts when I was working in transactional law, but don't remember the specific details, other than that the written agreement took precedent over any oral agreement, which was allegedly agreed to before the written contract was entered.
So if the alleged oral (gentlemen's) agreement was followed by a written agreement, it's my understanding of contract law that the written agreement overrides any oral agreement.
You might be able to sue for what's known as "specific performance" arising from default in a contractual agreement.
The kind of attorney to see is someone who specializes in contracts, which sometimes generally is considered "transactional law." It could also include the practice area of business law, which would be closer to your situation.