Is the Umrah valid if ablution is nullified during Tawaf, followed by the completion of the remaining rituals, and then the Tawaf is repeated on a later day?

If you suffer from waswasah (obsessive whispering), do not pay attention to it. However, if you do not suffer from it and you doubt that something has exited from you, then presume that your tahara (purity) remains, because certainty is not dispelled by doubt. If you become certain that your wudu' (ablution) was nullified during tawaf (circumambulation), then it is obligatory to repeat wudu' and tawaf, because purity is a condition for its validity according to the majority of scholars. You would also need to repeat the sa'i (ritual circumambulation between Safa and Marwah) because it follows a valid tawaf. Therefore, you must return to Mecca to perform tawaf and sa'i and to exit the state of ihram (ritual consecration) for Umrah. As for any haram (prohibited) acts you committed while in ihram due to ignorance, there is nothing upon you concerning them. As a precaution, a fidyah (expiatory ransom) is required for any damage caused. If you are unable to go to Mecca, then your ruling is that of one who is muhsar (prevented from completing the pilgrimage), and you may exit the state of ihram by slaughtering an animal and shortening your hair with the intention of exiting ihram.