With God's guidance, I say: Tayammum is invalidated by one of these two matters: First: An invalidator of ablution (wudu): Tayammum is invalidated by the original invalidator, even if the original was a full wash (ghusl); because the invalidator of the original invalidates its substitute, since it is a replacement, and the rule of the substitute is that it does not remain with the existence of the original: like fasting in expiation if one becomes able before completion. Accordingly: if one performed Tayammum for major ritual impurity (janabah), and then became impure again, he becomes impure but not in a state of major impurity, so he performs ablution if water is found and is not required to perform a full wash. Second: The removal of the excuse that permits Tayammum, even if during prayer: like someone who finds enough water for ablution if he is impure, or for a full wash if in a state of major impurity, whether owned or permitted and in excess of his need, he is required to buy water at its equivalent price or with a slight overcharge if he has money in excess of his need, and like someone who started praying with Tayammum due to illness, then was cured while in prayer, and like someone who started praying with Tayammum due to cold, then was provided with warm water while in prayer, and like someone who started praying with Tayammum due to the presence of a wild animal preventing access to water, then the animal left while in prayer, in all these cases his Tayammum is invalidated. See: Al-Ta'liqat Al-Mardhiyya p.38, and Sharh Al-Wiqayah p.110, and God knows best.