what ground may simons brothers challenge the agreement with ted?

Simon and his two brothers own the Family Business. Simon is the one who runs the daily operations. The Business has done well. Three years ago Simon hired Ted, and Ted has essentially taken over the daily operations. Simon has come to trust Ted and depends on Ted a great deal for advice and support. Simon's health is poor, and he wishes to retire. Simon has signed an agreement to sell Ted his (Simon's) share at a discount. Based on these facts, on what ground may Simon's brothers challenge the agreement with Ted