The credit invoice total should be changed to -$25 and the breakdown would be -$35 towards the return to vendor GL code - 5990 by default with the difference of +$10 to the markdown GL code - 5980 by default.
But . . . If those were in a previous month then changing them now won't affect the previous month.
you'd have to take the 'hit' in the current month by doing a GJ entry - perhaps +$1000 to the mark down account ( e.g. 5980) and -$1000 to the payables account.