Sheikh, who met Sarabjit in Lahore jail yesterday, in an email to the prisoner's daughter Swapandeep, said that her father had signed the letter addressed to Zardari for his mercy on the 65th Independence day of Pakistan.
Sheikh quoted Sarabjit as saying that he was very disappointed and saddened when the recent news about his release turned out to be wrong.
Sheikh's email to Swapandeep was forwarded to former Supreme Court Chief Justice Markandey Katju, who had strongly taken up the case of the release of veteran Pakistan scientist Khalil Chishty from a prison in Rajasthan recently.
Justice Katju wrote back to Sheikh last night, saying the conviction of Sarabjit was "flawed" and demanded his immediate release.
"I have carefully studied Sarabjit's case and am convinced that he was not guilty of the charge against him in the Lahore bomb blast case.
"... The main prosecution witness, Shaukat Salim, later retracted his statement and said it was given under police pressure," he said, adding "the other evidence was Sarabjit's alleged 'confession' but we all know how such 'confessions' are obtained in our countries (by third degree methods)."
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He said it was unfortunate that in "both our countries we often regard each other's citizens as devils, and I strongly believe that both Khalil Chishty and Sarabjit were victims of that mindset.
Justice Katju said he had sent several letters to Zardari in connection with Sarabjit's case, but has received no response.
He sent copies of his email to senators Babar and Iqbal Haider, as well as to Asma Jehangir, the President of the Supreme Court Bar Association and to Chaudhry Fawad Husain, the Special Assistant to the Prime Minister of Pakistan with a request that they should do whatever to obtain justice for Sarabjit.
He also sent a copy of his mail to the secretary of actor Salman Khan, who has organised a campaign for the release of Sarabjit.