Amount Adjustment on FIFO basis

peterpan 

let
    payments = your_TableA,
    items = your_TableB,
    f = (tbl) => 
        [recs = Table.ToRecords(tbl),
        payment = Record.FieldOrDefault(pr, Text.From(tbl[Invoice Number]{0}), 0),
        gn = List.Generate(
            () => 
                [i = 0, 
                paid = List.Min({recs{0}[Invoice Amount], payment}), 
                balance = List.Max({0, payment - paid}), 
                t = recs{0} & [Amount Paid = paid]],
            (x) => x[i] < List.Count(recs),
            (x) => 
                [i = x[i] + 1, 
                paid = List.Min({recs{i}[Invoice Amount], x[balance]}), 
                balance = List.Max({0, x[balance] - paid}), 
                t = recs{i} & [Amount Paid = paid]],
            (x) => x[t]
        ),
        back_to_tbl = Table.FromRecords(gn)][back_to_tbl],
    pg = Table.Group(payments, "Invoice Number", {"p", each List.Sum(_[Amount])}),
    pr = Record.FromList(pg[p], List.Transform(pg[Invoice Number], Text.From)),
    pi = Table.Group(items, "Invoice Number", {"pi", each f(Table.Sort(_, "Invoice Item"))}),
    expand = Table.ExpandTableColumn(pi, "pi", {"Invoice Item", "Invoice Amount", "Amount Paid"})
in
    expand

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: