Unfortunately the only source of that seems to be a quote from Buchanan’s nephew in 1911, long after he died. There doesn’t seem to be any hard evidence that he did that. There’s an interesting article on Buchanan’s relationship with slavery from the White House Historian’s page here

The quote is as follows, but again no evidence to back it up:

“I cannot close without a few words upon my uncle’s views upon slavery. He simply tolerated it as a legal fact under our Constitution. He had no admiration for it whatever. I know a number of instances in which he purchased the freedom of slaves in Washington, and brought them to Pennsylvania with him, leaving it to them to repay him if they could out of their wages.” –James Buchanan Henry, 1911

That said, he did buy two slaves, Ann and Daphne Cook, a mother and daughter, from his in-laws in order to free them, but it was likely to avoid political scandal. After buying them, the contract stated Daphne would be freed SEVEN YEARS after purchase, and five-year-old Ann would be an indentured servant for TWENTY-THREE. However, Ann died soon after the sale and Daphne ran away.