What is the ruling on the 'Isha prayer after finishing it and making the Tasleem, if a slight impurity (a trace under the fingernail) is discovered during the voluntary prayer performed thereafter? And can Ibn Baz's fatwa be relied upon in this case? And is there a possibility of the impurity transferring to the hair and ear?

If you did not know about the presence of impurity until after the prayer, then your prayer is valid. Your discovery of the impurity during a supererogatory prayer (nafilah) does not invalidate the obligatory prayer (fard), as the obligatory prayer concludes as soon as one makes the tasleem from it. However, remembering a non-pardoned impurity during the prayer invalidates it. A very slight impurity (like the head of a needle) is pardoned by some scholars; this refers to what is not excessive in one's perception, or if it was a drop or two. Therefore, we advise you to distance yourself from obsessive whispers (waswas), and you are not required to repeat your prayer.