Make bogus data as shown in the video here:
Depending on your data source, you can pivot your data when you edit your data source in Tableau. For data sources that don't allow this, you can ETL your data and pivot it before sending to Tableau. The effective result is that you have two rows per department -- One for money in, and one for money out. And you'll have only one department dimension, and that's what you'll fund along you r axis.