Sum/pivot problem with redundant data

[shelbygt500]

I have a challenging problem.

I have a table with invoices and reciepts and their amounts. I would like to sum their amounts and compare them.

However, the data is stored in several forms:
1 invoice (1 invoice amount) - 1 reciept
1 invoice (with several invoice amounts) - several reciepts
1 invoice (with several invoice amounts) - one reciept

Because of that pivot table gives inaccurate results.

How to get accurate summed data?