I think he's talking about wages. If the club won't pay sterling x amount what makes you think they will offer x amount of wages to another player if you sell Sterling?
I don't remember the wages we had offered in the past to certain players we bought with too much money(e.g Carroll, Torres, etc) but i think the problem with sterling is that he is asking for too high wage which in compare with his contribution and performance so far he does not deserve.(I am not saying he has been a bad player, he was great, but clearly not so good to demand such a wage.) With that in mind and the fact that Gerrard is leaving in the summer I think we have 1 big wage available to offer(the one that Gerrard was getting) so clearly not waste into Sterling but in someone new that will get us to the next level as a team. And it's easier to give that wage into a new recognized player rather than a 21 years old player, because then Henderson would come out and say I am the captain why is sterling getting that and I am not etc, and would have negative impact in the team balance....but if it is a transfer i believe it would be easier accepted. But yeah you are right... they might as well don't do it....just now is the chance to keep getting in the UCL since United is out of form and don't know when we will have that opportunity again so they should invest in that.