Skilled slaves were expensive to hire

"The commissioners got periodic lessons on how expensive a skilled slave could be. James Claggett, a local contractor, hired out his slave George to the commissioners for five dollars a month. But when the commissioners needed one of their scows repaired, Claggett sent them a crew of four men including "Negro Aaron" and "Negro Davy." They got paid as much as free white carpenters for the four-day job."

Quote from Slave Labor in the Capital, page 32

I made the top portion of the account scanned below one of the illustrations in the book. I didn't include the writing at the bottom because it shows that it took Claggett a year to get paid. That gives me the impression that the commissioners at first balked at paying and Claggett got a Montgomery County justice of the peace on his side to force them to pay. 

Thanks to John Sharp's work we know that Aaron Claggett appears on an 1808 payroll at the Washington Navy Yard, as a caulker and is listed as a slave belonging to James Claggett (Page 53, Sharp's African Americans Enslaves and Free at the WNY, draft provided by author).  That shows that while many skilled slaves personally profited from slave hire and bought their own freedom, many others didn't.  Despite his marketable skills,"Negro Aaron" who was a slave in 1795 was still a slave in 1808.

The "41" on the scan is my note. The document was cited in chapter 41 in Through a Fiery Trial.