Say this guy stinks up the league the first year and the team falters rather than conquers. The owner wants to renegotiate because the guy isn't playing like the owner thought he would. Sorry, pal, but that's what incentive clauses are for. You got the guy you wanted by guaranteeing the money instead of vesting it in incentives, otherwise somebody else might have gotten him. You've got to live with it.
Deals are deals.
The only instance I'd allow renegotiation is when a player voluntarily steps forward to take less money or spread existing money over a greater number of years in order to make room under the cap to bring in other players to increase the team's championship chances. Should have built that chance in. Sit out until the end of your contract. Deals are deals. They must be lived with.