I agree with the last bit about our abiity to close transfers but there is no evidence that, by signing Di Maria, we will not be able to sign other players.
You are looking at it from a personal point of view, i.e. if I buy a new car, I cannot go on holiday.
It is completely different from United's perspective. We are one of the top three most valuable clubs in the world, we have a large amount of income from sponsorship deals that far outweigh the expenditure on these players and there is also a very good incentive to invest in top players.
We have the money, we have the need and we can attract the players, so why not?
Whether we are actually able to get them, as our negotiation tactics and approach seem akin to a child in a toy store, is another question.