Do accounts for Morris's buildings show the use of slave laborers?

In Through a Fiery Trial, I cover the efforts of Robert Morris and John Nicholson to each build 10  brick houses just off South Capitol Street. They had to meet a deadline to prevent Daniel Carroll of Duddington from taking back the lots. Nicholson's correspondents wrote letters about their inability to pay workers and evidently they had to pay wages each day to keep laborers on the job. That might have made it difficult to hire slave. The lawyer William Cranch oversaw the building of Robert Morris's 10 houses. His few letters reveal nothing about the laborers he used, but his accounts do.

The accounts list charges for both the South Capitol Street project,  Square 651 and work in other areas like a house in Square 115 in the west end of the city. A laborer named Riley earned a $1.48 at 115 and a laborer named Carson $4 for digging a vault at 514. We can assume Riley worked alone, but Carson might have had a slave or two working for him. Some of the men listed as getting paid had a reputation for using slaves like Edward Burrows who got $10 and Robert Sutton who got $33 for digging a cellar. To me most likely indication of slave use is the $100 paid to Burrows for the use of laborers. He was paid in late November well after the shell of the buildings were done and Carroll's demands satisfied. The advantage to hiring slaves was that by tradition masters were paid at the end of the year or end of the quarter. So Burrows could hire slaves for work in August and not have to pay their masters until late November.

The accounts also give the names of contractors in 1796. Peirce Purcell, the head carpenter at the White House, was evidently selling bricks. The man who I think was the famous Philadelphia brick maker, Jonathan Kale, was also paid for something unspecified.