Calculate balance in Power Query with M - powerquery

Question

I have two tables:
The first holds the refunds sent to customers:
Customer ID
Refund ID
Total refunded
1
Ref1
$100.00
1
Ref2
$150.00
1
Ref3
$200.00
2
Ref4
$300.00
and the second links the above table with the relevant credit notes:
Customer ID
Credit note Allocation ID
Refund Amount Allocated
Refund ID
1
CnAll1
$100.00
Ref1
1
CnAll2
$100.00
Ref2
1
CnAll3
$50.00
Ref2
1
CnAll4
$100.00
Ref3
What I want to achieve is a third table with a combination of the two with the refund balance not allocated for each refund
Customer ID
Refund ID
Total refunded
Credit note Allocation ID
Refund Amount Allocated
Balance not allocated to any Credit note
1
Ref1
$100.00
CnAll1
$100.00
$-
1
Ref2
$150.00
CnAll2
$100.00
$-
1
Ref2
$150.00
CnAll3
$50.00
$-
1
Ref3
$200.00
CnAll4
$100.00
$100.00
2
Ref4
$300.00
$-
$300.00
Refund Ref1 is matched with the relevant credit note in full. The value of the credit note is = the refund. No balance not allocated
Refund Ref2 is matched in full as well against two credit note. The value of the credit notes is = the refund. No balance not allocated
Refund Ref3 is matched only for $ 100 because the value of the credit notes is $ 100. The balance not allocated is $ 200-100=$ 100
Refund Ref4 is not matched at all therefore the balance not allocated is $ 300
I cannot find the way to calculate the correct balance in all the scenarios above.

Answer 1

Its mostly a question of doing a few merges, then shifting the ID row by one for comparison
Load your second table as Table1 into powerquery
Load your first table into powerquery with code:
let Source = Excel.CurrentWorkbook(){[Name="Table2"]}[Content],
#"Changed Type" = Table.TransformColumnTypes(Source,{{"Customer ID", Int64.Type}, {"Refund ID", type text}, {"Total refunded", Int64.Type}}),
#"Merged Queries" = Table.NestedJoin(#"Changed Type", {"Customer ID", "Refund ID"}, Table1, {"Customer ID", "Allocated Refund ID"}, "Table1", JoinKind.LeftOuter),
#"Expanded Table1" = Table.ExpandTableColumn(#"Merged Queries", "Table1", {"Credit note Allocation ID", "Refund Amount"}, {"Credit note Allocation ID", "Refund Amount"}),
RefundByID=Table.Group(Table1, {"Allocated Refund ID"}, {{"Refund Amount", each List.Sum([Refund Amount]), type nullable number}}),
#"Merged Queries2" = Table.NestedJoin(#"Expanded Table1", {"Refund ID"},RefundByID, {"Allocated Refund ID"}, "Table1", JoinKind.LeftOuter),
#"Expanded Table2" = Table.ExpandTableColumn(#"Merged Queries2", "Table1", {"Refund Amount"}, {"Refund Amount.1"}),
#"Replaced Value" = Table.ReplaceValue(#"Expanded Table2",null,0,Replacer.ReplaceValue,{"Refund Amount.1"}),
//shift Refund ID down one row
shiftedList = List.RemoveFirstN(#"Replaced Value"[Refund ID],1) & {null},
custom1 = Table.ToColumns(#"Replaced Value") & {shiftedList},
custom2 = Table.FromColumns(custom1,Table.ColumnNames(#"Replaced Value") & {"Next Row"}),
#"Added Custom" = Table.AddColumn(custom2, "Balance", each if [Refund ID]=[Next Row] then 0 else [Total refunded]-[Refund Amount.1]),
#"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"Refund Amount.1", "Next Row"})
in #"Removed Columns"