The problem with your scenario is that people who had a lot of slaves also usually had a lot of land (that's why they needed so much labor to work it!) So your assumption that most of the wealth of slaveowners was tied up in slaves doesn't quite make sense. And it was more than an economic issue-for many slaveowners, it was a way of life, and one they were unwilling to give up.

It's probably safe to say that a buyout would've had to have been paid for by taxpayers; aside from the (as you acknowledge) huge amount of capital tied up in the slave economy, I don't know how many abolitionists would've been okay with buying freedom for slaves (which I'd note was as a general rule possible before the civil war - if abolitionists with the capital to do so had wanted to destroy the slave trade by buying up and freeing the slaves, they could have).

All that said, I think the dollar amount would have been unrealistic, frankly.

By your numbers, 25% of national wealth was tied up in chattel slavery.

By modern day standards, federal expenditures hover around 20% of GDP.

I'm not going to do that math, but it's safe to say it would've taken decades to pay off slave owners - at a time when the US did not have the borrowing capabilities it does today to take on debt.

Sorry, but your submission has been removed because we don't allow hypothetical questions. If possible, please feel free to rephrase the question so that it does not call for such speculation, and resubmit Otherwise, this sort of thing is better suited for /r/HistoryWhatIf.