I have never read anything that suggested US Grant as a slave owner or pro-slavery. Can you point us to your source please?
There are many sources - Just Google US Grant Slave.
Here are a few:
http://www.nas.com/~lopresti/ps.htm
http://www.american-presidents.org/2007/02/grant-was-slave-owner.html
http://en.wikipedia.org/wiki/Ulysses_S._Grant
(footnotes and references provided)
“A civilian at age 32, Grant struggled through seven lean years. From 1854 to 1858 he labored on a family farm near St. Louis, Missouri, using slaves owned by his father-in-law, but it did not prosper. Grant owned one slave (whom he set free in 1859); his wife owned four slaves (two women servants and their two small boys).”
http://www.vahistorical.org/lg/cw.htm
“Grant said little about slavery before the war. In 1859 he freed the only slave he ever owned; the domestic servants that his wife had inherited were beyond his control. Grant reenlisted in the U.S. Army not to free slaves, but to oppose secession. “I never was an Abolitionist,” Grant wrote, “not even what could be called Anti slavery.” But as he moved his army into Confederate territory and encountered thousands of black refugees, he concluded that “the North & South could never live at peace with each other except as one nation, and that without Slavery.” Once elected to the presidency, Grant worked persistently to protect the hard-won rights of the freedmen.”