Because of the way dollar adjustments are applied to forecasts, you can sometimes see an unexpected result when creating a dollar adjustment for a negative amount. If your forecast total did not decrease by the amount of your adjustment, then this could be why:
The forecasting tool will take the total dollar amount of your adjustment, then divide it by each interval (usually 30 minutes) during the range of time it is supposed to be applied to.
For example, if you apply a -$800 dollar adjustment to this location for the time 10:00 PM - 12:00 AM, then these are the intervals that will be adjusted:
Since there are 4 intervals, the adjustment will attempt apply $200 to each interval ($200 x 4 = $800). Since negative sales forecasts are not possible, and some intervals do not even have $200 forecast for that time, the calculation will zero out those intervals but will not apply the additional negative dollar adjustment elsewhere.
If you encounter this issue, it is best to either narrow the time frame that your adjustment applies to (i.e. avoid 24 hour adjustments) or to make a percent adjustment that creates a forecast that you feel is accurate.