Buyer support can’t do anything.
Your brother can simply call SELLER support and ask for a removal order to have the units sent back to him. 600 units will cost him $300 to be returned to him. They will either take it out of minutes owed, our charge his credit card. They will only ship it to a USA address. He can call the number above and ask to be transferred to the seller support side, then ask for an fba specialist. He can also send an email to seller-performance@Amazon.com.
The link for seller support is https://sellercentral.amazon.com/hz/contact-us?encoding=UTF8&ref=su_contactus_foot_xx&urlStr=%2Fgp
The only reason I can think of for Amazon not returning is if he was selling knockoffs or counterfeits. Those are illegal in all 50 states and Amazon would be obliged to have counterfeits destroyed.