Summerize rows differences in one table

This measure will return this table. You cannot get a table like you are showing above using the MTD values because your visual has no dates, so nothng for DatesMTD to work on.

Sales Difference = 
VAR varLastDate = 
    CALCULATE(
        MAX(Sales[Date]),
        ALLEXCEPT(Sales,'Date')
    )
VAR varPreviousDate = EOMONTH(varLastDate,-1)
VAR varCurrentDates = 
    FILTER(
        ALL('Date'[Date],'Date'[Month End]),
        'Date'[Month End] = varLastDate
    )
VAR varPreviousDates = 
    FILTER(
        ALL('Date'[Date],'Date'[Month End]),
        'Date'[Month End] = varPreviousDate
    )
VAR varCurrentSales = 
    CALCULATE(
        [Total Sales],
        varCurrentDates
    )
VAR varPreviousSales = 
    CALCULATE(
        [Total Sales],
        varPreviousDates
    )
VAR Result = varCurrentSales - varPreviousSales
RETURN
    Result

You can see my PBIX here with the date table I am using. 

If that is not what you want, please provide more info, but DATESMTD() requires a date in the filter context to operate on, and the result table you show above has no date in it.