Conditional Filtering Based on Slicer Selection

Hi grkm , I have achieved your requirement using a measure and a calculated table to make it dynamic. Have attached the DAX and the file along with this reply.  If you don't want to display the measure you can hide by shrinking that column in a table visual.

//Calculated Table
Slicer Table = 
var AllCities = SELECTCOLUMNS('City Data',"city_id",'City Data'[city_id],"city_name",'City Data'[city_name])
var LAggregate = DATATABLE("city_id",INTEGER,"city_name",STRING,{{0,"L Aggregate"}})
RETURN
UNION(AllCities,LAggregate)

//Measure
Measure = IF(
    SELECTEDVALUE('Slicer Table'[city_name]) = "L Aggregate" , 
    CALCULATE(COUNT('Branch Data'[branch_id]),'Branch Data'[branch_size] = "L"),
    CALCULATE(COUNT('Branch Data'[branch_id]),TREATAS(VALUES('Slicer Table'[city_id]),'Branch Data'[city_id])))

Thanks,
Jai Rathinavel