Spurs reportedly want more than the £55 million Utd are willing to offer, and that's why discussions have deadlocked this afternoon over the player's big-money move. The Standard claim Tottenham's interest in Frenchman Anthony Martial has strengthened Man Utd's position, and it may make Spurs more open to negotiating a fee.
Spurs recently offered Alderweireld a contract worth £120,000 per week to stay, but he's keen on the switch to Old Trafford. There's a very good chance José Mourinho's side will offer better money than the Belgian has been making in London.
Utd's position has been made even more powerful by the fact Alderweireld's contract expires in 2020. If they don't accept the £55m bid now and elect to hold on to the player, then Spurs can expect to receive far less than that sum next year as his deal ticks down.
For now though, they're standing firm. Man Utd haven't (yet) upped their initial figure to meet Spurs demands.