Is it permissible not to wash a dry, scaly spot on the foot during ablution if water increases its dryness?

"Nashaf al-jild" (skin dryness) is skin dehydration due to lack of water. If using water increases the disease and delays healing, then it is an excuse to forgo washing it. The sound part should be washed, then the affected part should be wiped if possible. Otherwise, tayammum (dry ablution) should be performed for it after completing the wudu (ablution). Ibn 'Uthaymeen said: "If there is a wound on some of the purification limbs, it should be washed with water. If washing it with water would affect it, then it should be wiped. If wiping it would also affect it, then tayammum should be performed for it." If the eye is afflicted with a disease that water would harm, it should be wiped if possible. Otherwise, the sound parts of the face should be washed and tayammum performed for the rest.