The 23-year-old is regarded as one of the best young defenders in Europe and has reportedly attracted a host of European interest.
However, De Boer told reporters: "That's maybe something for next year. I think he'll end this season at Ajax.
"We're very happy with him and don't want to let him go in the winter unless it's a super transfer fee, but I don't think that will happen."
Spurs are expected to move for the defender in January.