I'm not sure Southampton are in the strongest position on this.
Championship season starts this weekend, and you would presume that they would want time to reinvest anything they bring in. Yes, they could try and squeeze another £2 of £4 million out of us, but if they have players lined up, there comes a point where you may as well cash in. Replacements could go elsewhere, or the prices could go-up if Southampton are seen to be sitting on a lot of money from the Dibling sale.
Apparently the player isn't happy and wants to move on. So there is obvious a risk in keeping him. If for whatever reason, he doesn't leave this summer and they don't get promoted (which wouldn't be a surprise given how awful they were last season), his value isn't going to be any higher than it is now.
And if he's not happy, do they play him during August, with the knowledge that an injury could scupper any potential incomings they have lined up? Very risk to be starting a £40 million player in the Championship.