What is the ruling on someone who intended to perform 'Umrah, went to Makkah, but did not perform it and instead exited his state of ihram, then returned three days later and performed it, while not remembering to clip his nails and being unaware of the Sharia rulings?

Whoever intended Umrah but did not enter into ihram, owes nothing. As for one who has entered ihram for Umrah, he remains in the state of ihram and must complete it; it is not permissible for him to abandon it. He is liable for a compensatory sacrifice (fidyat al-itlaf) for every forbidden act he committed unknowingly. However, he owes nothing for acts of comfort that were committed due to ignorance of the ruling. Doubt about trimming nails does not necessitate anything unless he is certain or strongly believes he did it, though offering the fidyah out of precaution is better.