Good luck with the sale, Here in The Toronto Area I can not get rid of a Big Umbee for free!
Yours is an above average looking Dovii but I'm gonna give you my honest opionion , in my experience in Ontario there is very few if not none that will keep 2 fish in a big tank. You will get rid of him easy now at that size but he will probably go from tank to tank to fish store etc! IMO keep him in the Six footer, much better life then being tossed around, you have time! He ain't gonna grow any more for a long time! You obviously take care of him really good and IMO he would rather be in your care in a smaller cage then tossed around! Just my opinion!