I think we'd be silly to sell De Bruyne in the summer without taking advantage of the "farewell" bump; ala Klopp right now for Pool. I do think we should sell him to Saudi however, if he's open to the move. The money is just too good.
However, I don't think the Saudi clubs will lower their bid much, if at all, if forced to wait one more year. I imagine that once next season is heading into the business end, De Bruyne / City announce they're parting ways at the end of the season (or the season after if Kev is still up for it, not counting that out). This gives our fans and players the kick in the ass they need to find that extra drive to push over the line.
Knowing Kev, he won't love this approach, but the man deserves his flowers. I would expect applauses from all opposition club as he does his rounds after our away matches, knowing it'll be his last time gracing their pitch. Needless to say, the same love will be shown at the Etihad.