Agreed with Stuart, that the player is at the TDs mercy.
The oversized chip rule is one of those rules which I feel I am forced to ignore intent because of the language of the rule. Let's say the SB was only in for 100, and had a huge stack of 100s behind, and then threw out a 500 chip. If intent were the only consideration here, then this seems like a clear raise to me, otherwise wouldn't he just throw another 100 in? But the rule says call.
In this case, the SB is facing a bet, and he threw in a single oversized chip. I am not 100% comfortable with it, but I feel like the rule is clear, and I would probably rule this a call. If he had taken back the 500 and threw them both in simultaneously, then I would rule this a raise. Easier for player to verbalize his intention, for sure. I say this now, but who knows. It's highly situational, and a raise to 600 sounds defensible too.
On a related note, how would you feel if the SB was a beginner, asked the dealer "how much to me"? Dealer replies, "you are in for 100, it is 100 to call". Player says, "100 to call?" Dealer nods. Player throws in 500 chip. Does the prior conversation sway your ruling?