We won't always have that position, though. And the minute we falter and they finish above us, they'll be back insinuating that we owe them for letting Rashford/TFM/Schneiderlin/whoever move to us, while our own players will expect us to allow them to move to United, since we're clearly open to doing business with them. Saying 'when it suits us' won't really inspire affection in that scenario, and could be looked at as two-faced by the players who we refuse moves to.
Overall, I think the Bale situation was about as well managed in terms of the sale of one of our stars as it could have been. Sell abroad, and refuse equivalent or higher offers (I think United went to 100m) from within the league. Since the Berbatov sale, we've had almost no business of any kind with United, Chelsea, Liverpool and Arsenal (save for free transfers like Cudicini and Gallas, and weird deals like the one for Fryers), and I think it's for the reasons I listed above - personally, I'd like to keep it that way.
Rashford and TFM are promising young players, and maybe if Mourinho sells them somewhere else, we could then make an approach to buy them in six months to a year, as we did with Fryers. But buying directly from United? Nah, not quite yet.