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Hot sauce isn't the only thing that burns at Chipotle -- Frank Ocean just posted a photo of the fat check he sent to the Mexican Grill after the company sued him for $212,500 ... and on the memo, he wrote "F*** OFF."

Frank posted the image on his Tumblr account -- and a rep for Chipotle has already released a response, telling TMZ, "Thanks, Frank."

We broke the story -- Chipotle sued Ocean, claiming it paid him the $212,500 as an advance to record a song for one of its commercials. Chipotle claims Ocean bailed on the project because the Mexican Grill insisted on using its logo in its own ad ... even though Frank didn't want it ... the audacity.

The Chipotle rep tells us, they haven't received the check yet -- but they're grateful he ponied up.