DSO DAX Calculation - Countback Method

You can achieve countback by looping calculation inside sumx formula together with variables.

DSO (CB) = 

SUMX(
    // outer loop
    VALUES('Transaction'[Period]),
    VAR daysInMonth = 30
    VAR ocurrPeriod = 'Transaction'[Period]
    VAR ocurrARBal = CALCULATE(SUM('Transaction'[A/R]) , FILTER('Transaction', 'Transaction'[Period]=ocurrPeriod))
    
    RETURN 
    
    SUMX( 
        // inner loop
        FILTER(ALL('Transaction'[Period]), 'Transaction'[Period]<=ocurrPeriod),
        VAR icurrPeriod = 'Transaction'[Period]
        VAR icurrRev = CALCULATE(SUM('Transaction'[Net Sales]) , FILTER(ALL('Transaction'), 'Transaction'[Period]=icurrPeriod))

        VAR revSUMX =
                CALCULATE( 
                    SUMX('Transaction', [Net Sales]), 
                    FILTER(ALL('Transaction'), 'Transaction'[Period] >= icurrPeriod && 'Transaction'[Period]<=ocurrPeriod)
                )

        VAR revSUMXP =
                CALCULATE( 
                    SUMX('Transaction', [Net Sales]), 
                    FILTER(ALL('Transaction'), 'Transaction'[Period] > icurrPeriod && 'Transaction'[Period]<=ocurrPeriod)
                )

        VAR pleft = ocurrARBal - revSUMXP
        VAR cleft = ocurrARBal - revSUMX	
        VAR multiplier =
            IF (ocurrARBal > revSUMX, 1,
                Divide(pleft, icurrRev)
            )
        
        VAR result = IF (multiplier > 0 || cleft > 0,
            daysInMonth * multiplier, 
            0)
        
        RETURN
        result
    )
)