While a seller is required to disclose all pertinent, known facts to a buyer, it is not required to make any repairs. The sale of real property in California is strictly as-is, unless otherwise agreed to by all parties. In your case, the seller supposedly verbally agreed to pay for half the cost of a roof repair. It is more likely that the seller never agreed to make the repair but only agreed to credit half the cost of the roof repair back at COE, and somehow this was miscommunicated between the parties.
At this point the only question before you is do you still want the home; FNMA does not care what you do. They will absolutely let you walk away, and not lose a wink of sleep over it.