Q: John has £5. He knows that the book that he will buy in February will cost £4.50. In January he buys some apples for 50 pence. How much does John have left?
A: John has overspent by 25 pence. In December the Finance Department identified this as a budget with a projected underspend and accordingly removed £4.75 from the account without telling John that they'd done it.
Which is why Frog and Bronwyn are spending money they haven't got on an author event for World Book Day and not worrying as much as usual about how much it's going to cost.