I read somewhere, quite some time ago, that there was some reason to believe it was Castro that fingered Guevara because he wanted to rid himself of both an embarrassment and a potential rival.
El Jeffe took care of anybody who could become more popular than him. Remember General Ocho.
Castro basically sent him to Bolivia to theoretically start a revolution - but gave him no official instructions or any personnel or material support. He gave Guevara only two non-fucntion shortwave radios.
Castro figured, correctly, that Che the idiot would get himself killed or imprisoned overseas - much more convenient for Castro than killing or imprisoning him in Cuba.
When the Bolivians found Che, Castro had no idea where Che was or what he was doing - he couldn't have fingered him because all he knew was that Che was wandering around in the Bolivian countryside somewhere.