When did Abraham Lincoln use the phrase "As I would not be a slave, so I would not be a master?"
Historians know a great deal about Abraham Lincoln. Strangely enough, though, they do not know the exact source of the saying that you quote here. It is believed to have been written by Lincoln before he became president. The date that is generally given to it is August 1, 1858. However, even the most reputable of sources do not have any knowledge of the circumstances surrounding the quote. As you can see here and here, there is no attribution of the quote to a specific letter or speech. It appears in Lincoln's Collected Works, but without a clear origin.
So, the best guess is that he used the phrase on August 1, 1858. But the circumstances in which he did so are not clear.