If the MP3 is for piano solo, then I have absolutely no freedom of interperting the notes.
Every single note which is played by the pianist on the recording, should also be in the sheet music. Every note which ISN'T played, should not be in the sheet music.
Or did I misunderstand what you want to do here?
As for whether I did this before - yes, countless of times. At any rate, I'll send you a lo-res watermarked copy of the transcription for your approval.
If the transcription doesn't seem good enough, you pay nothing. Only if you are satisfied, will I send you a high-res copy of the music (for which you pay, of-course).