Is money given to an inspection officer after he has completed his work, which the giver claims is a gift in appreciation of his effort and not a bribe, considered lawful or unlawful?

There is no harm in accepting a gift from those who have been inspected after work is finished, provided it does not affect the job. However, it is more cautious to decline it, because accepting such a gift might lead to leniency in the future, thus becoming a forbidden bribe, the doer of which the Prophet, peace and blessings be upon him, cursed.