There are two ways to reduce a category value: By typing the exact number "917.52", or by letting the inline calculator do the work for you by entering "927.51-10", basically appending "-10" to the current value in the cell. The calculator is really useful for adding or subtracting odd amounts, and for performing specific math on category values that comprised of money carried over from previous months plus money that has budgeted in the current month.

Reducing the category value should toss the money back up to ATB.

Edit: I see that you already figured it out.