a word or term that encapsulates both the payor and payee.
I believe that would be party or party to transaction.
party a person or people forming one side in an agreement or dispute : a contract between two parties.
The New Oxford American Dictionary
party to the transaction - a party of people taking a role in legal proceedings
The Free Dictionary
payee - Party to whom a bill of exchange (such as a check or draft) is made payable.
Business Dictionary
A third-party transaction is a business deal involving a buyer, a
seller and a third party.
Investopedia
The party issuing a check is called the payor (in legal parlance) or
the payer (in common communication). The party whose name appears on
the check or to whom the check is issued is called the payee.
Bookkeeping