I don't massively follow the club ownership stuff these days, but he was at Strasbourg which has same owners right? Is there a world where that 6 years could include any team in their control - so if he fails they could just move him back to Strasbourg?
Anything is possible, but, theoretically, Strasbourg would be independently managed locally and they might not want to have a manager imposed upon them, and it's unlikely that there would be anything in his Chelsea contract guaranteeing he'd go back to Strasbourg as then Strasbourg would need to be party to the contract.
But a possible scenario is as follows:
1. Rosenior gets a big uplift in salary moving to Chelsea
2. It doesn't work out.
[These first two bits are pretty much guaranteed]
3. Rather than sacking him, Chelsea put him on gardening leave - so he's still their employee and they pay him as usual, but he doesn't have to fulfil his duties as manager
4. Strasbourg decide they want him back as manager (possibly being leaned on by BlueCo) and offer him a deal on lesser terms. Alternatively, AN Other club comes in for him. Normally, there'd be a fee for this but it's likely the fee would be minimal / nil as Chelsea just want rid.
5. Chelsea agree to pay Rosenior the difference between the Strasbourg salary and the Chelsea one, for whatever is left on his Chelsea deal (best guess, 6 years). This is either a negotiated lump sum or, more likely, month to month (better for cash-flow). I think they would take the accounting hit for the full cost when that deal is agreed, regardless of how the payments are structured.
End result is that Chelsea save part of the pay-off they would have had to give him (equivalent to the Strasbourg salary, plus possibly some employer social security savings) and Strasbourg get their man back on terms they can afford. Rosenior gets paid his Chelsea wedge regardless.
All of this will be complicated by bonus arrangements, and they'd need to do a deal on that. I'd expect Rosenior to be on a relatively low basic, but with big bonuses (which seems to match what they are trying to do with players).
That all assumes that the geniuses at BlueCo (or their advisers) have the foresight to do this. But it's not uncommon - I believe it's what happened when Pochettino left Tottenham. Part of the reason he was out of work so long was that he was still under contract so anyone poaching him would still have had to pay Tottenham a fee. Tottenham kept him under contract so they could claim a fee and potentially save his salary. But the longer he was out of work, the less they would earn.