The judgment overturns that of the Lagos High Court which sentenced him to death by hanging.
The presiding judge accused the lower court of being “stroked to secure a conviction by all means.”
Mr. Al-Mustapha was a former chief security officer to the late dictator, Sani Abacha.
He was sentenced to death on January 30 for conspiracy and murder of Mrs. Abiola.
Mrs. Abiola, 45, was shot in Lagos on June 4, 1996, as the lower court ruled, on the orders of Mr. Al-Mustapha.