Let's say you have $1m of spare cap space going into 2013.
That does not mean you necessarily had $1m of spare cap space over the course of 2012.
Do you see the distinction?
Yes, but I'm not sure you do.
If the Swans are paying their full COL, that's not going to change between 2012 and 2013. So our cap space can be thought of as Z+Y1-(X+Y2), where Z is the base cap, Y1 is the total COL, X is our base payments, and Y2 is the 10% extra we pay our players from the COL (which will be very close to equal Y1, and when the cap is maxed, will equal Y1). Now, we can't, at the end of this year, just choose to stop paying Y2 to the other players and direct it to a player like Tippett, as it's built into the players' contracts. Our cap space can increase, but only by reducing X+Y2 together. Furthermore, because Y1 and Y2 are basically equal, we can remove them from the equation, leaving our cap space as Z-X, which is the exact same as every other team in the competition.
Now, if we were somehow saving our COL allowance (not possible, given manager expectations and the minimum salary limit, but hypothetically), that would allow us to spend that money on a guy like Tippett, but it would necessitate that the COL is not part of the player contracts, or can be switched on and off at will (it most certainly can't). The only way to free up some on the spot would be to negotiate the contracts for players out of contract this year without the COL, which players and managers obviously wouldn't accept, and would only free up a small fraction of it. Therefore, under the assumption that either all players get it or none do, we would have had to be not using it at all to have that money lying around for Tippett. Therefore, we won the Premiership without it.
Now, the reality is the first example. We use the COL, 10% is added to every player's market value as a matter of course, to account for the expense of living in Sydney. In other words, if players don't get their slice of the COL, they look elsewhere. So there's no way to manipulate it to free up some cash for a big name. It's just not possible. But if it were, people would still have to choose between the Premiership having an asterisk, or Tippett's trade being unfair.