I say, and with God's help: It is permissible, if this is based on the report of a just Muslim doctor, and it is not a condition to fear loss; because it is permissible for him to perform tayammum if the water is sold at an excessive price, and the harm of the severity of the illness is greater than the harm of the increased price, thus tayammum is permissible for this reason; as Allah, the Almighty, says: {And if you are ill or on a journey or one of you comes from the place of relieving himself or you have touched women and do not find water, then seek clean earth and perform tayammum.} Surah An-Nisa: 43. Refer to: Sharh Al-Wiqayah p. 105, Fath Bab Al-‘Inayah 1: 110, and Al-Tabyin 1: 235, and Allah knows best.