[0.E.O]

The Union had a responsibility to remove their forts.

Did the South have a responsibility to pay for them first?

[CodeToad]

Nope. The primary reason I say this is that the federal government should always consider such things as forts as temporary, thereby not erecting permanent structures or expensive investments. The lands were taken from the State in the first place and should always be given back.

So no, there is nothing to pay for.