What food allergy causes a rash/hive only on the top of feet and hands?

None. Where the rash appears tells you nothing about food allergies. Hives can appear anywhere on the body. Eczema can appear anywhere as well but is more commonly seen in the elbow creases, behind the knees, at the wrists, ankles, tops of feet & hands, behind the ears, eyelids, & anywhere on the neck.
Hives hands / feet. The tops of the hands and feet are more susceptible than the palms and soles to having hives because the skin is thinner on the tops. It could be food, or medications, preservatives and other chemical agents and environmental causes. See an allergist to get tested.