Three years into his contract, what position is he going to be playing?
Exactly. It's a massive problem that's going to be hard for the two sides to resolve.
Xander has been a shortstop almost his entire career and is one of the top hitting SS in the game. He and his agent want him to be paid as one of the top hitting SS in the game using Tatis, Correa, Lindor, Turner, etc. as comps. That's a reasonable position to take in negotiation.
The Red Sox see that 1. Xander's defense isn't going to hold up at SS over 8 years and 2. They just drafted Meyer and Romero early in two straight drafts, have Downs in the system, have Trevor Story already on the major league roster, so by year 3 of the deal they see him as a third baseman and will want to pay him as a third baseman using Jose Ramirez and Austin Riley as comps. And that is also a reasonable position to take.
The two sides could meet in the middle to bridge the gap, maybe frontload the deal so he gets more in the years he's a projected shortstop and less when he converts to either 3B or 2B. But it's not a closed negotiation because Xander and Boras can and will go shop around to other teams who may not have a SS in the pipe who are looking for pure offense who will commit 8 years to him as a shortstop and probably give him close to the money he wants. So I can see this ending in an amical breakup for the Sox and Bogey and that doesn't mean that he's being greedy or that they're being cheap. It just seems like a reasonable and inevitable outcome of the negotiation process.