Acquiring Tanev's rights from Dallas gives Toronto the option of going to max 8 year term. Sounds crazy, but a longer team helps lure players from more tax friendly markets and lowers the AAV. Tanev is likely to sign in 5-8 yr range.
Correction. Post deadline the player has to be on a teams reserve list, so Tanev can't get more than 7 years from Toronto. The Leafs made the trade to jump start a negotiation to sign Tanev