Can "vorhin" be used instead of "von vorhin" in this sentence?

It would make a big difference in terms of meaning:

• "Lena ging mir voraus zum Ausgang, wo der Verkäufer von vorhin stand"

  This refers to the seller. "The salesman from before."

  (Lena went ahead of me to the exit, where the seller from before was standing.)

• "Lena ging mir voraus zum Ausgang, wo der Verkäufer vorhin stand"

  "... where he was standing before."

  (Lena went ahead of me to the exit where the seller was standing earlier.)

In both sentences the part ", wo der Verkäufer (...) stand" refers to the exit. It is a (local) relative clause like this:

„Zukunft ist der Ort, wo wir den Rest unseres Lebens verbringen.“ (Woody Allen)

The subordinate clause after the comma describes the place more precisely.

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Theoretically it would be possible to say something like that:

"Lena ging mir voraus zum Ausgang, wo der Verkäufer von vorhin vorhin stand."

(Lena went ahead of me to the exit where the salesman from earlier was standing before.)