Something similar happened to me once. I posted a note in the transaction that it was an error and stated "I authorize Venmo to reverse this transaction." I also stated that under no circumstance would I initiate a payment myself.

I also emailed Venmo. Venmo support told me to initiate a payment back. I told them I absolutely wouldn't do that and I was shocked that they suggested it. Venmo then reversed the transaction.

Venmo is a terrible company, their support is terrible, and they are actively encouraging their users to get scammed.