ПОДАЙ ПАЛЕЦ, А ЗА РУКУ САМ ВОЗЬМУ

Cf: Give a dog a finger and he will want a whole hand (Am.). Give him a finger and he will take a hand (Am.). Give him an inch and he'll take a mile (an ell, a yard) (Am.). Give him a ring, and he'll want your whole arm (Br.). Give knaves an inch and they will take a yard (Br.)