No, you are never obligated to sell to anyone, including the city, for less than--or for that matter, even for equal to or greater than--asking price; you decide which offers, if any to take.Yes, a city could try to take property by "eminent domain," but to do so, they have to bring and win a legal action against you, proving they need the land for some legitimate reason, and they have to take it for market rate. Until and unless they bring such an eminent domain action and prove their case in court, it is voluntary on your part whether to take their offer or not.