Fielder Balk??

[radwaste50]

Almost all codes have a statement similar to the NCAA code I copied here

At the start of or during a game, all players of the defensive
team except the catcher must be in fair territory when the ball is put in play.
Being in fair territory means that a defensive player must have at least one
foot placed in fair territory.
a. The catcher must stand with both feet within the limits of the catcher’s
box until the pitcher releases the ball.
PENALTY—It is an illegal pitch if no one is on base and a balk with a
runner(s) on base.

Now you said he only had one foot in foul territory if the other is in fair ............
could be OOO you can also use the catcher to reel the first baseman in before balking the player in foul territory