More like a sale-and-leaseback - but selling the order book slots and effectively leasing back via the difference in payments on the eventual leaseback of the aircraft.
There is no "profit sharing" as such, they (maybe) got less than the full value of the slots (value to Boeing), and may have ended up paying back more in extra payments on the lease than they got in cash - but that is common in leaseback deals, particularly if the seller really needs the money (and they apparently did).
The only legal worry I can think of would be the are laws about "undervalue" transactions before an insolvency, but the "asset" here isn't easy to value at all. This isn't like selling off (and leasing back) the company premises to your mate and only getting half the market value, so half the value has been put out of reach of the creditors. I am not sure the order book slots would have _any_ value at all in the event of insolvency, therefore the creditors would appear (as others have already commented) to have more money than they would have had if the slots hadn't been sold.