Cumulative Subtraction using Criteria

Hi alessiomissio 

There isn't that much data to test on but here it is based on what's available. Please note that a proper date column had to be used as cumulative can't be meaningfully applied to text (your month column)

Cumulative Amount t1 = 
CALCULATE (
    SUM ( T1[T1 Amount] ),
    FILTER (
        ALL ( Months ),
        Months[Start of Month] <= MAX ( Months[Start of Month] )
    )
)

Balance = 
SUMX (
    ADDCOLUMNS (
        ADDCOLUMNS (
            SUMMARIZE (
                FILTER ( T1, T1[T1 Amount] <> 0 ),
                'Key'[Key],
                Months[Start of Month]
            ),
            "@Cumulative Amount", [Cumulative Amount t1],
            "@T2 Amt", CALCULATE ( SUM ( T2[T2 Amount] ) )
        ),
        "@balance",
            VAR _bal = [@T2 Amt] - [@Cumulative Amount]
            RETURN
                IF ( _bal > 0, 0, _bal )
    ),
    ABS ( [@balance] )
)