You re-sign Fuller on day one. $12M/yr isn't unreasonable, which would be in the Darius Slay ballpark.
Slay's stats before contract extension: 4 INT's, 37 PBU's, 123 tackles, 0 FF's over 3 seasons
Fuller's stats heading into FA: 8 INT's, 41 PBU's, 156 tackles, 3 FF's over 3 seasons
Granted numbers aren't everything but does give you an idea how productive Fuller has been.
He had a relatively solid rookie year under garbage Mel Tucker. He then started slowly in a new 3-4 defense with Fangio but finished the second half of 2015 on a positive note. 2016 misses whole year and thrown in doghouse. Bounces back this season with a career year, testament to his resolve to silence the doubters.
Fuller is just entering his prime, the best is yet to come. Slay just had his pro bowl breakout season this year at age 26; Fuller will be 26 next season.
Good CB's are hard to find and letting a good one in Fuller walk would be criminal. Pay the damn man.