I think a swap deal (Guehi) + money would poss be the best option for both parties.
There's a 20% sell on fee for Guehi (supposedly) so we're losing at least £10m on any offer (if we get offers above £50m for him). I think we've accepted that he'll leave in the summer as he'll be in the last year of his contract, so we will need a replacement CB.
Chalobah is not as good as Guehi, but has advantages over trying to find another CB. He's a good Premier League CB, still young, already knows the team & setup here and therefore would settle quickly. Essentially, we know what we'll get from him and I still suspect that there was more to come from him when he was in our team. It's waste of talent for Chelsea to have him on the bench, but does mean that his value shouldn't sky rocket (like it did for Gallagher when he was here). Chalobah + £20m maybe a reasonable deal to be done. That would value Guehi at £50m-£60m, which maybe right for a player in the last year of his contract.
It obviously all hinges on whether there's a bidding war for Guehi, which there may well be. There are many teams that would want him due to weaknesses or players likely to leave. Liverpool, Newcastle, Tottenham, Man U, Man City & possibly other European teams. I doubt we'd get more than £50m-£60m for him (I maybe wrong). Therefore, we could go out and buy another CB + potentially another player for £20m-£25m each, but there's no guarantees they'll settle or be anywhere near the level needed.
The question is whether to go for 'tried & tested' and bank potentially £20m or take the gamble of a bidding war, potentially get more money, and look for a bargain like we got with Lacroix or a risk like with Riad (who I still think will be a great player, but we haven't been able to use him this season).