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.
You can't say it's a mistake from a business standpoint, but of course it's a big mistake from a rivalry point of view. Tottenham won't be selling him to a rival club because that would just kill themselves when Kane plays against them over the next season, but if this mindset will change from a business perspective.
You have to sell your players to get money, even if they are star players. Manchester City sold Gabriel Jesus to Arsenal, West Ham sold Declan Rice to Arsenal, Tottenham sold Kyle Walker to Manchester City and that is because they are doing business for their own benefit. But of course, regarding rivalry then you have helped your rival to be better based on the quality of the players.