Using a filter with no relationship

Here is one way to do this.  It assumes there is no relationship between the two tables, but it could be modified if that is the case too.  I added an Index column in query on your Deposits table to be used in the measure, to get the total amount of previous deposits (DepositOrder column).

Spend FIFO =
VAR totalspend =
    SUM ( Spend[Spend] )
VAR thisdeposit =
    IF (
        HASONEVALUE ( Deposits[Deposits] ),
        AVERAGE ( Deposits[DepositOrder] ),
        MAX ( Deposits[DepositOrder] )
    )
VAR thisdepositamount =
    SUM ( Deposits[Amount] )
VAR previousdeposits =
    CALCULATE (
        SUM ( Deposits[Amount] ),
        ALL ( Deposits ),
        Deposits[DepositOrder] < thisdeposit
    )
VAR remainingamount = totalspend - previousdeposits
RETURN
    IF (
        HASONEVALUE ( Deposits[Deposits] ),
        IF ( remainingamount > 0, thisdepositamount - remainingamount, 0 ),
        SUM ( Deposits[Amount] ) - totalspend
    )

I used different data so the #s don't match yours, but here is what it looks like in use.

If this works for you, please mark it as the solution.  Kudos are appreciated too.  Please let me know if not.

Regards,

Pat