Final answer:
The hired man's one accomplishment was gaining control of his earnings for the first time when he found employment stowing a sloop, marking a significant moment of independence and self-reward.
Step-by-step explanation:
According to the text, the one accomplishment of the hired man, which is to say the narrator's own achievement, was taking control over the fruits of his labor for the first time. When the narrator found employment stowing a sloop with a load of oil, it was the first instance where the reward of his work was to be entirely his own, signifying a pivotal moment in his journey from enslavement to autonomy.
He expresses profound joy and satisfaction from this experience, underscoring the psychological and emotional impact of earning for oneself after a lifetime of servitude where others claimed the rewards of his efforts.