Adelaide need a $10 note.
Carlton have 2x $4 = $8
Sydney have $10 note
Adelaide give $2 + Carlton's $8 to Sydney and receive a $10 note.
Get it?
All apart from the exchange of players and the player you picked up, which was gifted to you by the AFL.
The trade ends up being Carlton paid pick 13 and and an SANFL player for pick 40. There's your missing $2.