Here's the other scenario, which makes every bit as much sense, IMHO:
1) The Red Sox didn't want to pay above a certain amount, and they didn't think Mookie would accept that amount.
2) By trading him before 2020 they got some talent for him, and they removed a $27 million salary obligation and were able to get under the tax threshold by getting the Dodgers to also take on $15 million a year for Price.