Discount Reversal gives you the sum of the discount amount applied over all voided invoices ie. (Voided at date filter applied) and credit notes. The AR Aging Balance report is based on the invoice date and not the service period defined for each line item.
Therefore, for examplean invoice generated in the month of November can have a line item that has the service period mentioned for the previous or the forthcoming month. Multiple line items may create a discrepancy with summary and AR balance.
To overcome this and match the invoices export to a line item report, you can use the VLOOKUP feature from google sheets/excel that lets you categorize the invoices and line items from the CSV file. This way you can have access to multiple line items for different months.
The common entity shall be Invoice number in both the sheets. Using this you need to create a column in Line item for invoice dates and then you need to run your analysis. This would ensure that the discrepancy mentioned above does not occur and you can provide the required data at the line item level as well.