PHILADELPHIA — A letter that documents President Franklin Pierce's pardon of a black man who harbored slaves went up for sale Monday.

The 1854 presidential pardon, which is valued at $75,000, grants clemency to Noah C. Hanson, who was convicted three years earlier of stowing two slaves in a hiding place under the kitchen floor of his employer's Washington, D.C., home.

The document is currently the only known presidential pardon of a black man convicted of harboring slaves, according to The Raab Collection, a dealer based in suburban Philadelphia.

"This rare manuscript highlights the important role that African American people in the North played in the Underground Railroad and the risks they took to free slaves," said Steven Raab, founder of The Raab Collection, owner and seller of the document.

Stanley Harrold, a professor of history at South Carolina State University who has written a book about abolitionists in the nation's capital that includes the Hanson case, said he knew of no other pardon of its kind.

"There was an earlier pardon by President Fillmore of two white men imprisoned in D.C. for helping slaves escape, but as far as I know the Noah Hanson one is the only presidential pardon for a black man," he said.

Hanson, a free black, was arrested after he was discovered hiding the slaves, who belonged to a South Carolina congressman, in the home of his employer — himself a Southerner.