The following steps are demonstrated in the attached workbook. This is an example of showing Discount as a measure when only 1 field is selected in the Region filter and Sales as a measure otherwise.

1. Click Analysis > Create Calculated Field. Save the calculated field named as Values including the following formula.

   IF {COUNTD([Region])}=1 THEN [Discount] ELSE [Sales] END

2. Place Region to rows and filters.
3. Right-click Region in the filters pane > Show Filter.
4. Again, right-click Region in the filters pane > Add to Context.
5. Place Values to Text in Marks card.

Result:

Discount value is displayed when a field (East, West, etc.) is selected from the Region filter. 

Sales value is displayed when [All] is selected from the Region filter.