All clubs can spend whatever they can in whatever way they see fit. Spurs finished a single place above United last year, having spent god knows how much on AVBs pay-off and a heap of players who failed to live up to any of the valuations (e.g. Lamela). Some shocking transfers, but all within their means. United, also, bought Fellaini for which there is no answer to opposition fans ridicule, other than to say "yep, shit transfer, shit player". The onus is on the players being ineffective, the fee being secondary.
However, City and Chelsea have clearly doped and it's only in this context that I think it fair for other fans to raise eyebrows and discuss the ridiculous nature whereby an owner can come in and spend a ton of money that the club itself has failed to earn. They are the Lance Armstrongs of the football world.
United is a massive club. They can spend 60 mil on Di Maria. Woodward and co. can even get pissed and spend 100mil on Akinfenwa, still wouldn't matter (financially...). We care about winning, something that we haven't been that good at lately. This is a positive step. Long may it continue.