The Cameroon international is demanding a cut of the transfer fee that took him to Inter this summer and has decided to take the matter to the High Court.
"We were surprised," said Begiristain. "It is an unpleasant fact because we thought that everything had ended well. I do not understand this complaint and it is clear that we will end up in court.
"Eto'o had not communicated anything to us."
Asked if Eto'o will be warmly welcomed on his return to the Nou Camp for their Champions League clash, Begiristain added: "I do not know if he will now be well received."