The issue of slave trading was of great contemporary significance during the period in which Mansfield Park is set. However, while it is acknowledged that the main events of the novel take place at some point between 1803 and 1813, there is no consensus as to the exact period. It is not entirely clear, therefore, whether Fanny is asking this question before or after the passing of the Slave Trade Act of 1807, which abolished the slave trade (although not slavery itself*) throughout the British Empire. We are also left uncertain as to the stance of the different characters on this particular issue.
* The Slavery Abolition Act, which abolished slavery within the British Empire, was not passed until 1833.