Tebas, 62, reminded the La Liga champions of the 1:1 rule, which means they can only spend as much on salaries as their income allows in order to register new players.
He also confirmed that Athletic have lodged a complaint over Barcelona’s pursuit of Williams, 22, with Deco publicly stating their desire to sign the winger.
Speaking to EFE, Tebas said: "Athletic Club are complaining because Barcelona have publicly stated, if the statements attributed to Deco are true, that they want to sign a player.
“According to FIFA regulations, he is in a protected period, and we don't have the authority to interfere in that. What we will do, as we have always done, is that Barcelona will have to comply with the regulations to sign players, be it Nico Williams, (Erling) Haaland, (Lionel) Messi... whoever it may be. And we won't change anything."