property not for sale was sold

My parents were selling their home on seven acres. The realtor pulled all property in dads name. There was 1 acre 2 miles away not for sale. We told her and she took it out of the paperwork. After mom and dad signed the purchase agreement, we found out that the 1 acre was added in the "further conditions" section. It was hand written under the statement that the house was to be professionally cleaned. I first contacted the purchaser 1 year later(when it was time to pay taxes again). He didn't know that he owned it. He didn't even know where the property was. He said he would sign it over to me if I repaid him the cost of taxes for that year. I said not a problem(taxes are less than $50 a year on that property). In the mean time, my cousin wanted to put her horse on that 1 acre. I told her what was going on and to wait until it is in my name(parents name...whichever). She went to the purchaser and offered him $1000...he accepted. Can I get this property back for my dad? The realtor called the purchaser and he admitted to her that it was wrong and he knew he shouldn't have that 1 acre. What can I do?! Please help.

You can tell them to undo the transaction or you may sue them all for wrongful possession, fraud, misconduct etc. And whoever first included it/sold it/ can be liable for negligence.

Thank you for the reply. Do you know what forms I need to get started? Our financial situation is hindering the lawyer process. Is this something I can even do without a lawyer?

1 Acre is too small for a horse.