Amount Adjustment on FIFO basis

peterpan 

I know you have posted this question under Power Query but I would like to see if you are interested in a DAX solution, if so, add a calculated column in Table B, using the code below :

Amount Paid = 
VAR __InvNum = 'Table B'[Invoice Number]
VAR __InvItem = 'Table B'[Invoice Item]
VAR __InvAmount = 'Table B'[Invoice Amount]
VAR __TotalPayment = SUMX( FILTER( 'Table A' , 'Table A'[Invoice Number] = __InvNum ) , 'Table A'[Amount] )
VAR __CumAmount =  SUMX( FILTER( 'Table B' , 'Table B'[Invoice Number] = __InvNum && 'Table B'[Invoice Item] <= __InvItem ) , 'Table B'[Invoice Amount] )
VAR __RemAmount =  __TotalPayment - __CumAmount
RETURN
   IF( __RemAmount >= 0 , __InvAmount  , MAX( 0,  __InvAmount +  __RemAmount )  )

Output: