MILAN, May 14 (Reuters) - A Milan arbitral tribunal ruled in favour of Blackstone Group Inc in a disputed sale of the headquarters of RCS, rejecting all claims advanced by the Italian publisher, a court document seen by Reuters showed, confirming what sources had said earlier on Friday.

RCS Mediagroup, which publishes the influential daily Corriere della Sera, launched arbitration proceedings in late 2018 to nullify the 2013 sale of its historic headquarters in central Milan to Blackstone, saying the U.S. investment firm had paid too low a price at a time when RCS faced financial difficulties.

Blackstone, which paid 120 million euros ($145 million) for the offices, had in turn accused RCS of falsely claiming it still owns the building and of improperly blocking its sale to Germany's Allianz.

The sale of the property "was nothing more than a commercial negotiation" in which the buyer "legitimately" tried to achieve the "most advantageous terms of sale" without evidence of "any undue pressure on the other party," the arbitration award said.

RCS was not immediately available for comment.

"The panel of judges fully recognised the validity of the contract and the fairness of Blackstone's behaviour during all the sale process," a source close to the U.S. fund said.

Friday's final decision by the tribunal follows a partial sentence published in May last year ruling that the sale was valid.

The ruling had requested two expert surveys on RCS's financial situation in 2013 and on the property's market value at the time, to assess whether there had been any damages for RCS and to what degree.

The U.S. investment firm filed two lawsuits in New York which were put on hold pending the outcome of the arbitration in Italy.

It is seeking up to $600 million in damages from RCS and from its chairman and main shareholder Urbano Cairo, who took control of the company in 2016, a source close to the fund said at the time.

RCS's financial report, published on the company's website, shows it has not set aside risk provisions for the legal dispute.

(Reporting by Claudia Cristoferi and Alfredo Faieta in Milan Editing by Giulia Segreti and Matthew Lewis)