The POS API returns two fields:
[CLIENT_TRANSACTION_ID, 123456789]
[SERVER_TRANSACTION_ID, 123456789]
The documentation suggests using the Transaction API which is deprecated to lookup further details.
Is there a method without using the Transaction API to find the payment using Payments.GetPayment() (with some of the returned data as above)?
Currently looks like it needs a call to Transactions.RetrieveTransaction() (deprecated) to get the payment id (tenders / id) then a call to Payments.GetPayment().
The transaction_id that is returned from the POS is the is equal to an order_id. You can use the RetrieveOrder to get the order with the transaction_id. In the order response there will be the payment_id and the tender_id which you can use to GetPayment.
@accolades The true retirement date hasn’t been set at this time. We will definitely be sending out notifications about retirement of the Transactions API. To ensure you get notified of updates, new features, and functionality with Square APIs please periodically check our Release Notes or subscribe to our Developer Notifications.