Kessel isn't signed by Boston. He's a restricted free agent. If the Blues send an offer sheet to Kessel, Boston has the right to match it. If they don't match, the Blues send a 1st, 2nd and 3rd round pick to Boston.
Kessel was asking Boston for $4.5M per season.
I'd pass on both deals.
That doesn't mean a trade isn't in play. We should ship them picks, players, or prospects for the rights to Kessel. Then if he signed an offer sheet with another team then WE would get the 1st, 2nd, and 3rd round picks. However if Kessel doesn't get traded it would almost certainly be a sign and trade deal in which contract terms are agreed upon before the transaction is processed. Happens all the time in the NBA and to a certain extent the NFL.