This sounds like this should be done during the data preparation in the Staging Engine before it gets to the Cube. Either in the Import or Validate step or both. By the time the data records are validated and are prepared to load to the Cube, the data records at the base level should already have the absolute values logic applied for their amounts. Once it is loaded to the Cube, it is the raw base level data that is sacred. No manipulation needed and in the case of any audit with this data, there isn't a question why the integrity of the data changes in the Cube.
The raw data in the Cube should be kept sacred and should be the source of truth for your data. The Reporting Engine has the ability to show the data differently such as Henning mentioned above. Using Math.Abs works for Dynamic Calcs for Reporting and for GetDataCell whether in a Cube View or a formula. However, any massaging that needs to be done to the data should be considered during data preparation in Stage.
