CUSTOM Rollup with a Hierarchy

We had a scenario where the user want to see average on the DETAIL rows and the SUM of the averages on the TOTAL row in a matrix. The 'Calendar'[Full Date] column was on the rows and it was the lowest level of the hierarchy which include week, month, etc.

The 'Calendar'[Full Date] column represents a single day.

HASONEVALUE will return TRUE for a single day and FALSE otherwise. When FALSE, use SUMX to iterate over the days and calculate the SUM of the averages.

Here is the measure:

'Financial Summary'[Average Sales (With Rollup)] = 

VAR AverageSales = 
    AVERAGEX('Financial Summary', 'Financial Summary'[TransactionAmount])

RETURN

    IF (
        HASONEVALUE('Calendar'[Full Date]), 
        AverageSales, 
        SUMX(VALUES('Calendar'[Full Date]), AverageSales)
    )