Why is it hard to believe that a 'Spurs said raise your bid?
United offer ~10 million, Spurs tell them to up their bid to ~20 million ?
United offer ~10 million, Spurs tell them he's not for sale, United might make another improved offer but Spurs said he's not for sale.
Maybe because Spurs don't want to sell him

And equally, if they are willing to let him go for a large sum, they don't want to let Utd know they're open to selling him. If Utd want Modric so much that they're willing to pay the sort of sum Spurs will demand then they're not going to be put off by Spurs saying he's not for sale.