Slavery and Christianity in colonial America is a complicated issue. Liberals like things to be simple but they seldom are.
George Whitefield could be seen to justify it but I think it was more a matter of trying to make the best of a situation he had little control over. He simply suggested using it as a means of freeing blacks from the barbarism of Africa. He was given a slave (Surrey) who he freed immediately yet took her in to live with his family where she could be cared for as she was educated.
Whitefield later became a staunch and outspoken abolitionist.
Phyllis Wheatley was a global superstar poet and a slave. She traveled to Europe and met heads of state. She could have remained in Europe as a free woman but chose to return to America. She wrote the Eulogy for George Whitefield's funeral. Her poetry took a dim view of slavery and racism yet praised God for having been brought out of Africa.
An example
Twas mercy brought me from my Pagan land,
Taught my benighted soul to understand
That there's a God, that there's a Saviour too:
Once I redemption neither sought nor knew.
Some view our sable race with scornful eye,
"Their colour is a diabolic dye."
Remember, Christians, Negroes, black as Cain,
May be refin'd, and join th' angelic train.
Selected Sermons of George Whitefield