The move means that Nihat would miss Sociedad's crucial final two league games as they bid to clinch the Primera title.
The Basque club are enraged as title rivals Real Madrid have already been assured that their players will not be asked to play in the tournament as the Spanish league season comes to a thrilling climax.
Sociedad president Jose Luis Astiazaran has now written to the Turkish Federation asking them to reconsider their decision.
"Real Madrid and Valencia players will not play for their national team in Confederations Cup," he declared. "Nihat is very important player for us."
Meanwhile, Turkish club Fenerbahce are planning a swoop for Nihat's Sociedad strike partner Darko Kovacevic.
The Serbo-Montenegrin hit-man is still part-owned by Serie A outfit Lazio, and Real are struggling to find the finances to complete his permanent capture.
Primera rivals Valencia are also keen to add the powerful forward to their ranks, but it now seems Fener have joined the chase and Kovacevic admitted he would not be averse to a move to the Istanbul-based giants.
"Fenerbahce is great club in Turkey. I can prefer them, but they have to deal first with Lazio," Kovacevic is reported as saying.
