this is probably the original article from which the Eurosport article derives.
From what I understood, the Basque region could be used as a "Spanish tax heaven" just like Monaco or Cayman Islands by using certain methods.
The problem is he doesn't live in the Basque region, neither does his family. Arantxa Sanchez had the same kind of problems some years ago.
Yannick Noah also is in conflict with the French tax administration about where he lived in 1993 if I remember. For French players the usual tax heaven is Switzerland : usually they live there while they're still playing and then come back to France when they have stopped their carreer.