We can't just make someone else's fish!

(noun): A saying dating back to medieval Europe (apx 1100 C.E.). Originally meant to denote one fishmonger's unwillingness to copy his competitor's sign (and thus claim that he sold the same kind of fish), the saying has come to refer to any general unwillingness to copy someone else's work, specifically when requested to do so by a customer.

"I was wondering if you could make me some copies of this licensed product."
"Sorry but they own the rights to it. We can't just make someone else's fish!"