Dealer says "check" but the player never checked

[mblax10]

This hand occured playing 2-5 NL. The opponent in the hand is as nice and honest a guy as you'll meet, so I can almost guarantee he wasn't trying to pull any shananigans.

I go heads up to the river with the player sitting to my immediate right. He's led the action and there's apx. $130 in the pot. I make my baby flush on the river and he's first to act. With chips in his hand he thinks for a little bit. I'm watching him and the dealer says check.

Hearing this I quickly grab a stack of chips and cut out a $50 bet. As my chips go in the middle, opponent says "I didn't check." I finish cutting out my bet and he says "OK, leave it out their" and than he thinks and raises another $100. I quickly call and he shows a bigger flush.

I mention to him that the dealer said check, and ask how much he was going to bet. He said he was going to bet $100 and offers to give me $25 to split the additional money I lost on the river. I take it instead of making a fuss or calling the floor. If the floor comes over what would the ruling be?

This was the dealers last hand, the new dealer witnessed the hand and later verified that action happened as I described it.