The ball was fair.
Here's what Jeter had to say about the play in the fourth inning (via NYDailyNews):
"Yeah, it was kind of a odd play for me, because I was playing in at third for a bunt and ended up going back for a pop up. The pop up started going foul so I was running in foul territory then came back, so it never even crossed my mind that it was fair. I almost gave it to a fan. I’m glad I didn’t. I thought it was foul at the time because I was in foul territory."Oh Derek.
Brett Gardner tried to make a sliding catch on the play, but missed. Thinking it was foul Jeter took his time retrieving the ball and when he did looked to give it to a kid. Garnder and pitcher David Phelps yelled at him to throw it in before he gave it up and he did.
And a simple pop-up become a triple. Nice Jeter. Nice.