This letter is not written by an eighteenth century slave owner - it is one of these things that has been started as a (clever) joke, and taken on a momentum of its own because ullible people don't check historical facts.
Before you go crackers, I'm not claiming that slave-owners didn't use deliberate tactics to control the people they enslaved - just that this document was not written by any of them.
Black people actually weaken their case if they base it on such shaky evidence, especially when there is a wealth of genuine evidence of slaveholder brutality.
Something which is rarely addressed is "slave-owning" in the guise of market forces. Read Orwell ("Road to Wigan Pier"), London ("People of the Abyss") and Sinclair "The Jungle"). Here we see "slaves" who are in a worse state in many ways than those owned by plantation owners - those at least had a value to their owners. People who are kept at the bottom of the social/work heap can be overworked and abused with impunity by their "owners", who dump them to starve when it suits, and make sure that they never get a job again of they try to better their conditions. They did not even have the value of a slave.
I am not trying to minimise or excuse the slave trade - it was a horrific part of world history, and still continues today. It was the worse because it was carried out largely by people who claimed to be Christians, but who broke every one of Christ's admonishments. I cannot even imagine the psychological as well as the physical damage which would be done to anyone who can be bought and sold, and bred or refused the opportunity to breed, housed and fed or neglected and starved like an animal. It is beyond hideous.
What I am trying to do is ask people to put things into context and base their research and argument on a sound basis. No-one needs to make up horrors - they are well documented.