There's a lot of uneducated opinion here about Alderweireld and Spurs so I'll fill you in. Before Xmas last year Spurs entered into discussions with the player about extending his contract. Toby indicated to Poch and the club that he would like to stay and extend if his financial expectations were met. Poch went to Levy and told him that if the club could offer what Toby was looking for, he would commit. This salary was high but not too high, around 120k - 150k a week. At that time, no player at Spurs not even Kane were on that figure. Toby gave Poch his word he would commit, so Poch convinced Levy to offer the wages, which they did. Alderweireld's agent /dad, the player, his wife, or a combination of all three, decided that now they weren't happy with what they originally said they wanted, and wanted more.
This fecked off Poch immensely, particularly as Poch has gone to Levy and convinced him to offer the player to break our wage structure. This is why Toby was basically hardly picked for the first team even when he got back fit.
Spurs subsequently gave Kane and others much higher contracts in the summer. Toby, on realising interest in him wasn't as much as he expected it would from the major spanish 2, only United showed serious interest, indicated he would be open to staying if he could then be paid the new top salaries that Kane was now on. Poch wanted him out though, trust had been broken, and Levy was in no mood to offer the guy such a high contract after the way he behaved.
That is basically the jist of it.
He will be a United player in a couple of days no doubt.