Its not clear in codes. However according to High Court rulings, seller should mention any additional payments (including tax) in the agreed amount in the beginning of the agreement and he can not increase this later on his own. Eg. this is mentioned Yargıtay's (Turkish High Court) ruling 13.10.2004 E.2004/19-456 K. 2004/531 here:
http://www.turkhukuksitesi.com/showthread.php?t=18181