Thank you very much! After I did a lot of research, I found out that

For some reason, PayPal maintains two different ids pointing to the same completed transaction. One is used for Seller/Merchant and the other is used and displayed for Payer. PayPal does not expose Payer’s transaction id in the view of Merchant and even we can not retrieve it by any Rest API from my system.

Fortunately, although PayPal does not let us know the transaction ids of Payers but they allow us to search a transaction by both Merchant’s transaction id and Payer’s transaction id pointing to the same transaction.