While I disagree in comparing arb salaries (~8mil) to FA offers (~18mil), the most recent precedent of your comparison would be Grandal. He drew 7.9mil in his last arb season, declined the Dodgers Q-offer, Signed by MIL as FA for 1/18.25mil w/mutual option (Cots).
With Pederson, it’s power gets paid. If he matches his 2018 & 2019, i’d expect a similar scenario / FA salary, unless it’s multi-year, which decreases the AAV. Last season Pederson’s ISO was 11th in MLB w/at least 500 PA’s.