Threatening letter buyer's attorney for breach of contract

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I'm unlicensed architect, I sold a duplex with entitlements for a lot split and infill single-family. I created the construction documents for the proposed design but chose not to pursue development and sold the property instead. The design of the single-family was not part of the purchase and sale agreement for the property or advertised as such. As per the contract I delivered copies of all the drawings in my possession but did not release the rights to use them or agree to transfer the digital files. 

The contract was for the purchase and sale of the property and nowhere in the contract does it include assignment of intellectual property rights. I offered to cooperate with the buyer and share the drawings for a reasonable fee. 

Today, I received a letter from his attorney requesting I assign all intellectual property rights to my design or agree to damages ($250,000) or to mediation. 

So, who here has gone to mediation? From my perspective, the buyer would rather compensate their lawyer for my design than to compensate the designer. 

Happy Friday, thank you for your thoughts.