Then how will you delete the negative feedbacks ? Will you hack the amazon system
Without the account access of the person, who gave negative feedback, this cannot be done.
It might be possible by contacting Amazon customer service on merchant's behalf but that would require knowing at least the details of the account, which again is not advisable to provide to a known scammer even if the scammer doesn't require a payment in advance. Such details could be used to compromise the account later on.
But likely it's just a plain scam like bL4nkcode said above.