Difference between two table values with different dates

Hi jamuka 

Please try below measures:

Sales version 1 = 
var selected_calendar_month=MAX('Calendar'[StartofMonth])
var selected_actuals_month=CALCULATE(MAX(Sales[Date]),ALL('Calendar'))
var selected_forecasting_month=CALCULATE(MAX('Forecast'[Forecasting Month]),ALL('Calendar'))
var totalsalesactuals1=CALCULATE([Total Sales],ALL('Calendar'),FILTER(ALL(Sales),Sales[Date]=selected_forecasting_month))
RETURN totalsalesactuals1

Forecast version 1 = 
var selected_calendar_month=MAX('Calendar'[StartofMonth])
var selected_forecasting_month=CALCULATE(MAX('Forecast'[Forecasting Month]),ALL('Calendar'))
// var totalsalesforecast=SUMX(FILTER(ALL(Forecast),Forecast[Date]=selected_calendar_month && 'Forecast'[Forecasting Month]=selected_forecasting_month),Forecast[Total Forecast])
var totalsalesforecast1=CALCULATE([Total Forecast],ALL('Calendar'),FILTER(ALL(Forecast),Forecast[Date]=selected_calendar_month && 'Forecast'[Forecasting Month]=selected_forecasting_month))
RETURN totalsalesforecast1

delta version 1 = IF(ISBLANK(Forecast[Forecast version 1]),BLANK(),
[Sales version 1]-Forecast[Forecast version 1]
)

Below is the screesnhot:

Sample PBIX.

Please give kudos or mark it as solution once confirmed.

Thanks and regards,

Praful