If a person performs their prayer under a concession with a valid excuse, and the excuse exists at the time of performing the prayer, there is no obligation to repeat the prayer if the excuse ceases to exist, whether during the prayer time or after it. There are many examples of this, including:

• If someone prays using tayammum (dry ablution) and then finds water within the prayer time, they do not have to repeat the prayer. This is supported by the ḥadīth of Abū Sa῾īd al-Khudrī: two men went on a journey, and when the time for prayer came, they had no water. So they performed tayammum and prayed. Later, they found water within the prayer time. One of them repeated the prayer and wuḍū᾽, while the other did not. When they mentioned this to the Messenger of Allāh (peace be upon him), he said to the one who did not repeat, “You have followed the Sunnah and your prayer is sufficient for you.” He said to the one who repeated the prayer and wuḍū᾽, “You will have double the reward.” [Reported by Abū Dāwūd and An-Nasā᾽ī].
• If someone combines prayers in advance (Dhuhr with ῾Aṣr) and then is returns from his travel during the time of the second prayer (῾Aṣr), they do not have to repeat it according to the majority of scholars.
• If someone shortens their prayer and then returns before the end of the prayer time, they do not have to repeat it either.

The key consideration is that the obligation is fulfilled with the existence of a valid excuse.

Fatwa issued by Dr. Khālid Naṣr