107.3c

If a spell or activated ability has an {X}, [-X], or X in its cost and/or its text, and the value of X is defined by the text of that spell or ability, then that's the value of X while that spell or ability is on the stack. The controller of that spell or ability doesn't get to choose the value.

Spoils of War.

Note that the value of X may change while that spell or ability is on the stack.