Final answer:
Master Thomas leased Douglas to Mr. Covey due to his dissatisfaction with Douglas's behavior and the hope that Covey's known ability to 'break' and discipline slaves would correct Douglas's conduct.
Step-by-step explanation:
Master Thomas leased Douglas to Mr. Covey because he found Douglas unsuitable for his purposes. Douglas's city life had a negative impact on him according to Master Thomas, making him unfit for work and prone to negative behaviors like allowing the horse to run away. Douglas was also subjected to severe whippings without any change in behavior. To address this, Master Thomas decided to have Douglas 'broken' by Covey, who had a reputation for effectively disciplining young slaves. Master Thomas's decision was influenced by his displeasure with Douglas's disobedience and the hope that Covey's methods would reform him.