Calculated Field won't format

I'm fairly certain this is a simple fix, but for the life of me, I can't figure it out. I'm trying to fix a custom order form that was built. We have a Unit Price and a Markdown Amount field that calculate into a Net Price field (Unit Price minus Markdown). Somehow the order form stopped calculating the Net correctly. I've re-added the calculated field to the report and it's working, but the format is not correct. When I pull up the Report Field Options for any other field that's formatted how I need the Net Price to be, the Format Field options are able to be selected. If I pull up the Report Field Options for the Net Price field, those options are grayed out. I know what one I need to use, I'm just unable to choose it. 

I apologize if this is something really basic, but this is my second time working in report writer and I'm learning as I go.  I appreciate any help!