If your dad made a verbal agreement, it would be very difficult to prove such agreement took place, unless obviously there are more witnesses other than yourselves, the inheritors. If you accept the inheritance, then you assume the assets left by your dad. However, as universal heirs, you also take over his debts, if any, and the obligations or contracts he entered into, unless it is a personal obligation, such as a contract to, say, elaborate an artistic piece of work.
If the person with whom your dad entered into the sales contract sues you, he will have to prove the existence of the agreement in order to enforce it, and this could be difficult. But bear in mind that as heirs who have accepted the inheritance, you now assume the position of your dad.
