The Dutch midfielder is believed to be in talks with the Turkish outfit after refusing to take a paycut with the Nerazzurri.
Sneijder was present for Inter's 2-0 win over Pescara last night, where he told Sky Sport Italia that he is "considering his options".
"It is a big decision I have to take, so I don't think it will be made by Monday," said Sneijder. "There are still two weeks to go before the end of the transfer window, so there is time. I am relaxed and I'm considering my options."
However, Terim has now warned the 28-year-old that he must be committed to the club if he is to move to the Turk Telekom Arena.
"If Sneijder wants to come to Galatasaray, then he must see us as a first choice," Football Italia quotes the former Fiorentina and AC Milan coach as saying.
"This club is not anyone's plaything. Those who play here do not see Galatasaray as an alternative, but as a first choice. Otherwise, one must step aside."
Galatasaray are currently top of the Turkish Super Lig.