Mr. Spence hit it right off the bat when he wrote, "Secession, if lawful...". One side of the equation held that the southern acts were not lawful so the offer to pay, if it ever was made, would be moot. The first side had no interest in selling. It can also be argued that since the British government never recognized confederate sovereignty then they did not consider the secession lawful either.
The impression exists, that the people of the South proposed, from the first, to pay for all Federal property, and sent commissioners to Washington to arrange this.
That can be argued. The commissioners were sent to obtain recognition from the Lincoln government, and only once that was obtained to negotiate on issues of disagreement. Without the first condition being acceptable then the second part of the instructions are meaningless.
As for your Ministry of Propaganda request for a citation of International Law...
Don't have one, do you? Hence the lame attempt at insult.
One side of the equation held that secession was lawful. One side of the equation does not determine what the law is. One side may use military force to impose its will upon the other and then declare itself to be virtuous and right. That particular argument is called "might makes right."