Legal tender is about a currency being valid and sufficient for the payment of **debts**. It does not apply to spot exchange.
There is no economic, ethical, or legal justification for what he did. He stole strawberries and left an ungratuitous gift in stead. Asshole move. https://www.youtube.com/watch?v=whwyo5Vi4RA
If the merchant doesn't want dirty coins, then so be it.