Which payment processor offers line item capability?
I don't think there is a payment processor the will do what you want, it needs to be handled on the CiviCRM side.

If you create a price set with the membership fee as one field and the contribution amount as second field, you assign different financial types to each component of the transaction.  So although it looks like one contribution, the amount for each financial type will be recorded separately in the line_item table.

If you install the Extended reports extension and use the Contribution Based Line Item report you can get a breakdown of income by financial type.