Русско-английский словарь пословиц и поговорок
ЧТО ОБЕЩАНИЕ, ЧТО ЗАРОК НЕНАДЕЖНЫ
You can never be sure if someone will keep his word or not. See Обещанного три года ждут (O)
Cf:All promises are either broken or kept (Am.,Br.). It's one thing to promise, another to perform (Am.,Br.). Promises are like good piecrust: easily broken (Am.). Promises are like pie-crust, made to be broken (Br.). Promises are like piecrust: they are made to be broken (Am.)