Why would they be "taken" to the North? What right would the North have to forcibly displace free men?
Rather, with something like $3 Billion in monetary capital from Compensated Emancipation floating around the South and looking for Labor, it's probable that most blacks would have remained in the South and gotten jobs. Employed in agriculture again for the most part, sure; but as free men.
By the same token, what right would the North have had to negotiate a price for free men? They wouldn't have exactly have been free if the government could freely buy and sell them.
What would have happened if government had bought the slave's freedom and then turned their back, leaving the former slaves in the south? I guess we can never really know, but my guess would be that history would have unfolded much as it did after Reconstruction, except that it would have been even worse for black people.
If blacks had been suddenly freed without the Civil War taking place, they would have found themselves at the mercy of a strong, powerful, well financed Southern establishment which would have had no reason at all to treat them any differently than before.
Without the Civil War, the South wouldn't have been devastated. They would have maintained their economic power and capital, while the blacks would have had nothing. They would have still had all of the money, all of the property, and all of the political power and structure. My guess would be that former slave owners would have quickly developed some sort of debt-bondage or truck system which would have economically enslaved the workers.
In the post-War South, devastated and weakened in every way, all of the might the federal government could bring to bear wasn't enough to come close to neutralizing the former power structure. After the war, Congress had to put the Confederacy under the rule of the US Army in an effort to rebuild local governments. Those who had held positions in the Confederate government were denied the right to hold office or vote again. Even with all of that, as soon as Reconstruction ended, black people were again disenfranchised with Jim Crow laws as those formerly in power began to bounced back.
There is no reason to think that if the South had been allowed to maintain itself, that blacks would have been treated decently, since they weren't treated decently after Reconstruction. I guess they wouldn't have had to have moved to the north, but not much would have changed if that hadn't been included in any hypothetical deal.