Slaves were becoming less valuable economically. Machines were starting to replace human labor, and had considerable benefits. They didn’t get sick and die, nor runaway; you only had to feed them when in use. Slavery would have died out in time when it simply became financially advantageous to mechanize the labor. Mechanized farming would have ended slavery just the same as it replaced horses and mules.
Link to time line of mechanized farming in America.
http://inventors.about.com/library/inventors/blfarm1.htm
Rather than buying the slaves, the north should have offered discount pricing on mechanized farming equipment to southern farmers. This would have killed off slavery faster than anything else.
The reasons the south was using slaves was because, at the time, it was the cheapest way to farm. Give them a cheaper way to farm and what need would they have for slaves?