This approach totally makes sense from the club's perspective, but is not at all great for the player.
I feel the same way.
I don't want this to be a club that stifles the ambitions of its players. If he has a chance to play in the Premier League, and he can go at a time that allows us to find a replacement, then I wish him well.
As a general club rule, I'd make it clear to any players joining the club that they won't be allowed to leave in the summer under any circumstances. Winter, feel free; summer, absolutely not.
Euro teams usually look for full time solutions in their offseason. The winter window is generally for desperate moves/impact additions, so the demand for players is significantly lower. It's not like a Prem team looking to sign Long now will wait for him for 4 months until January; they'll just sign another guy.