Your tables have to be linked someway, for example, by month (Table1 to Table 2, 1 to many)
Then a new measure:
Sales diff = CALCULATE(SUM('Table 2'[Sales]))-CALCULATE(SUM('Table 1'[Target]))
Then you'll get desired amount for all months or for selected month only.
For example, if you make a bar chart visual with month number as x-axis, and put this measure in the values, you'll get what you want