Please help me to solve that! Thank you !!!

Saul had been Ben's corporate lawyer for more than a decade. Ben's daughter, Sylvia, who is in first year law, hopes to join Saul's law firm upon graduation. Ben owes Saul $50,000 for professional services. Saul suggests to Ben that the account could be settled if Ben were to transfer his condo to Saul. The $50,000 debt would represent Saul's down payment and Ben would carry the balance of the price, $200,000, by way of a vendor take-back mortgage. At first, Ben hesitated, but reflecting upon his need for Saul's legal expertise and his daughter's career, he agreed. A few months later, Ben's son David, who was to inherit the condo, learned of the transaction and persuaded his father to sue to recover his inheritance. a. What is the most likely cause of action that could be used by Ben to recover the condo? What would Ben need to prove to be successful? (4 points) b. What possible arguments could Saul use in defense? (2 points)