It's certainly possible the Sox limit an offer to three years, maybe even for $20mill. I'd be surprised if that was the best offer he received. And I agree, he goes for the best offer, regardless of where it's from.
Dombrowski will absolutely make an attempt to sign him, and might even make the best offer. But if that best offer is in the neighborhood of all the speculation, I think the ramifications going forward could be very negative for the future of the Sox. This team has a young and deadly core, and that core should be kept intact as long as possible. And if that means letting an occasional older and pricier talented player leave via free agency, then it needs to happen.
I think re-signing Sale after 2019 would be a mistake, too, especially after he started having injuriy issues this year (although they were probably not as bad as his limited playing time might make one think. There was definitely some coddling involved.) But if you think Kimbrel is going to be tough to replace, what about Sale?