Purity: Invalidation of Tayammum Upon Removal of Excuse

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 708:

If someone performs tayammum because he did not have water or some other legitimate excuse, then once that excuse expires, his tayammum becomes void.

-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)