Here's the key bit from the description --
"Going forward clubs may use a portion or all of their allotted Targeted Allocation Money to convert a Designated Player to a non-Designated Player by buying down, on a prorated basis, his salary budget charge to at or below the maximum salary budget charge. If Targeted Allocation Money is used to free up a Designated Player slot, the club must simultaneously sign a new Designated Player at an investment equal to or greater than the player he is replacing."
So even if we could use that money to buy down Mix, we'd have to sign a DP to replace him. And considering we've used all three spots already, we can't fulfill the second "must" of that clause.
So, from my reading, this money is useless to us outside of buying down Mix's contract when we're replacing him with a DP. I'm sure they'll make an exception for us, but I can't think of another use for it for us in the next couple years with Pirlo, Lamps, and Villa locking down spots.
Once those three are gone, though, this will really come in handy.