It is mathemagically gone!

But really, it is a misdirection, and a definition reframe.

The amount paid was 25 + 2 = 27
not 27 + 2 ≠ 30 (the 30 stopped existing half way through the story, or you could use 25 + 5 = 30, or 30 - 5 = 25, but you can't just drop the 5)