The transaction id (txn_id) in Paypal IPN is not the same as on the invoice?

I'm using PayPal on a website that I'm currently working on and everything works correctly: user is sent to paypal, pays, and comes back and IPN validation is working perfectly.

The only thing is that I noticed, using the sandbox, that the transaction number given to the buyer is not the same as the one I get in the IPN data (txn_id). I'm wondering how am I supposed to help a customer on my website if he gives me the confirmation number he got on his invoice and it's not the same as in PayPal's system? Is there something that I missed in my IPN scripting that would allow me to get that same information? I didn't see anything about that in the documentation and I'm probably not the first developer who notices that...

Thanks for any kind of 开发者_运维百科help you can give me.

A seller's transaction ID is different from a buyer's transction ID, since they're two different actions (one debit, one credit).
See from PayPal IPN unique identifier