Relative Date Function - IF Past do 'X', IF Present do 'Y'

hi, jtownsend21 

This looks like a measure totals problem. Very common. See this post about it here: https://community.powerbi.com/t5/DAX-Commands-and-Tips/Dealing-with-Measure-Totals/td-p/63376

Also, this Quick Measure, Measure Totals, The Final Word should get you what you need:
https://community.powerbi.com/t5/Quick-Measures-Gallery/Measure-Totals-The-Final-Word/m-p/547907

If not your case, please share your sample pbix file for us have a test.

Best Regards,

Lin

I found the solution. It is a 3 part solution: 

First, reconcile the values from previous months: 

*Reconciliation = 

VAR _REVENUE = 
    SUM(Revenue[Revenue])
VAR _INVOICES = 
    SUM(Invoice[Amount])
VAR _DATE_DELTA = 
    MAX('Date'[FirstDayofMonth])-DATE(YEAR(TODAY()),MONTH(TODAY()),1)

RETURN
    IF(
        AND(
            _DATE_DELTA < 0,
            _INVOICES = 0
        ),
        _REVENUE,
        IF(
            AND(
                _DATE_DELTA < 0,
                NOT(ISBLANK(_REVENUE))
            ),
            _REVENUE - _INVOICES
        )
    )

Second, add the reconciliation to the invoiced amount: 

*Correct Value = 
SUM(Invoice[Amount])+[*Reconciliation]

Third, create a measure to total them when multiple months are selected: 

*Correct Value Total = 
IF(
    HASONEVALUE('Date'[MonthYear]),
    [*Correct Value],
    SUMX(
        VALUES('Date'[MonthYear]),
        [*Correct Value]
    )
)

Thanks for the links v-lili6-msft  the answer was in there.