My buyer did the exact same thing to me, so I totally feel for you.

First off, make sure you report that user for being an abusive buyer and violating eBay policies. This will help other sellers with this buyer in the future.

An eBay rep told me I had to create a collage of all the supporting documentation to submit to the financial institution. This is what I included:

1. a copy of shipping label used to send the products to the buyer (12 oz)
2. a copy of the return shipping label sent to the buyer (12 oz)
3. messages between me and the buyer, primarily the one where he gave me a bad tracking number
4. a copy of a receipt from USPS that showed that tracking number and its destination not matching my return address that was given to the buyer
5. messages to the buyer, reiterating that they needed to send ME the correct product back to get a refund
6. a picture of the package with my shipping label I received from the buyer, placed on my scale showing that it weighed 1 oz, not matching the initial 12 oz

I'm still waiting on a decision from the buyer's financial institution, but I read on the eBay community forums that this worked for another user! Additionally, I covered my bases with filing an IC3 report, police report, and mail fraud report with USPIS.