Transaction fee

Hi, I would like help with the article use in the following example. My colleague and me are looking at the breakdown of a transaction. He asks: "What is this number?" I explain:

"This is a transaction fee".

"This is the transaction fee."

Is either article correct in this case? I think it is, but there is the following difference:

a transaction fee - I am not sure that you know that this transaction should have a fee, but it does and that's it.

the transaction fee - I think you know that this transaction should have a transaction fee, and it's this number here.

Right?