The official Herald newspaper said prosecutors had indicated to a magistrate court on Tuesday they were unable to proceed with the case against Samkange because of a lack of evidence.
Samkange told a court hearing last month that Zimbabwe's immigration department had frustrated his efforts to bring a witnesses into the country.
Samkange's witness, an Equatorial Guinea national who was said to have been living in asylum in Spain, was to have testified on prison conditions and torture in Equatorial Guinea but was denied a visa to enter Zimbabwe.
The lawyer is appealing against a court ruling to extradite Mann to Equatorial Guinea to face charges of plotting a coup against Teodoro Obiang Nguema Mbasogo, the president.
Mann himself was arrested in Harare in March 2004 and in September 2004, a Zimbabwe court found him guilty of trying to purchase weapons without a licence.
He was branded as the ring leader in the coup plot and was held along with 66 other men, who served lesser sentences.
Having served his sentence, Mann was arrested last week under an immigration warrant for deportation to Equatorial Guinea.