I don't really understand your game.. If you need to set lettersMatch variable per each box, then you need an instance variable. If you need to count successful matches, use a global variable.
You can add a pair of instance variables startX and startY to the box, set them to box.x and box.y on start of the layout. When the box is dropped and there is no match, return its position to box.startX, box.startY