You may want to use the variable:
L_UNIQUEIDn
(Optional) Transaction-specific identification number for tracking in an accounting system.
Character length and limitations: 30 single-byte characters. No whitespace allowed.
Ref.
https://developer.paypal.com/webapps/developer/docs/classic/api/merchant/MassPay_API_Operation_NVP/
This value can be then collected using Instant Payment Notification (IPN) service, please check its functionality here and the MassPay values you can collect with IPN are listed here.
For L_UNIQUEIDn, the variable returned with the IPN POST will be:
unique_id_x
For Mass Payments, the unique ID from input, where x is the record number. This allows the merchant to cross-reference the payment
Length: 13 characters