What should one who suffers from a scalp allergy and cannot shave or shorten his hair for 'Umrah do?

Shaving or shortening the hair is one of the obligatory rites of 'Umrah. Whoever omits it must offer a fidyah (expiation), which is a sheep, or one-seventh of a camel, or one-seventh of a cow, to be slaughtered in Makkah and distributed among the poor of the Haram. He would be sinful if he omitted it without a valid excuse. However, if he shaved his head before 'Umrah and there is nothing left to shave, then he is not liable for anything and can exit the state of ihram after the sa'i.