Running Total with If

// This will work OK only IFF for each
// company there exists AT MOST 1 day
// in the Payments table.

[Cumul Payment] =
var __lastVisiblDate =
    MAX( Date[Date] )
var __onlyOneCompanyVisible =
    // Company should be a dimension
    // that joins to Payments, so please
    // do yourself a favour and make your
    // model into a proper star schema.
    // This code assumes that Company
    // exists only in your fact table
    // Payments but this is NOT how it
    // should be. If you have Company
    // as a dimension, you'll change the
    // code below to
    // HASONEVALUE( Company[CompanyID] )
    // instead.
    HASONEVALUE( Payments[Company] )
var __result =
    if( __onlyOneCompanyVisible,
        var __weOwnTheInvestment =
            CALCULATE(
                SELECTEDVALUE( Payments[Own?], 0 ) = 1,
                // This is where you have to make
                // sure that the assumption from
                // above is observed. Otherwise,
                // the value of CALCULATE will always
                // return False.
                Date[Date] = __lastVisibleDate
            )
        RETURN
        if( __weOwnTheInvestment,
            CALCULATE(
                SUM( Payments[Payment] ),
                // Date must be a date table
                // in the model marked as such
                // for this to work OK.
                Date[Date] <= __lastVisibleDate
                
                // If you want to only sum up
                // the payments where [Own?] is 1
                // then you have to add as a filter
                // this condition to this CALCULATE:
                // Payments[Own?] = 1.
                // But if you do this, you'll only
                // get a running total for the
                // payments that you own, not the
                // true running total. But maybe this is
                // what you want... Nevertheless, the code
                // only displays the total if the
                // max day for the visible company
                // has the flag set to 1 regardless
                // of the method of summation.
            )
        )
    )
return
    __result