They may have done just that. I believe it's the payment of his remaining salary, though I could be wrong.
I don't know what Maniatis' contract was, but I can't imagine Tzandaris was on some absurd salary. He also signed with them two years ago if I recall, so if his contract was up this or next summer, and it costs Olympiakos 350k EUR to kick him out this year early, it's not a big price to pay by their perspective.