Exactly.
If a player has 4 years left, and is projected to be worth 12 fWAR over that timeframe, and each fWAR is valued at $8mill. And his projected salary is, say, $1mill for the first year, $6mill in the second, $9mill in the third, and $12mill in the 4th of those 4 years. Then his surplus trade value is (12 * $8mill) - ($1mill + $6mill + $9mill + $12 mill) or $96mill - $28mill, giving him a final surplus value of $68mill. Their system is a little more complicated, but those are the basics of it...
And as those performance projections change, and those salary projections change, and as those years pass, they need to be updated accordingly.