They don't want to sell Harry Kane to their rivals. I don't think money is the problem.
I understand why Tottenham are reluctant to sell Harry Kane to another English club.They know very well that he is a good player and selling to another english club will only mean that they are empowering their rivals. Chelsea do not think this way, that is why they have made the mistake of selling Kovacic to Manchester City. Kovacic will be brought back to his best by Pep, and he will become a very good player for Manchester City that can pair with Rodri in their midfield. Chelsea made a mistake.
Tottenham are just being unreasonable. A team like Manchester United is not a rival to Tottenham. There's nothing wrong with selling their player to United. If every team decides not to sell their player to other teams in the league how will the league look? Tottenham are ciy rivals with Arsenal so it can be understandable if they don't want to sell their player to them. Manchester United and Liverpool are also rivals so they will most likely not exchange players.
Whether they sell Kane to a rival or not, it doesn't mean anything.
Chelsea did not make any mistake. They have gotten the players they want. Man City would have still gotten another player in place of Kovacic even if Chelsea did not sell Kovacic to them. You don't expect the teams in the league to not do business with each other.