A peace deal with the leftist National Liberation Army (ELN) would cap an earlier deal with the larger Revolutionary Armed Forces of Colombia (FARC) to seal an end to a half-century civil conflict.
The government and ELN have said they are ready to start talks on February 7.
They agreed to talks on condition that the rebels free former congressman Odin Sanchez and that the government grant pardons to certain imprisoned ELN members.
"The protocol for his release has been initiated," Defense Minister Luis Carlos Villegas said on Radio Nacional.
"So I hope Mr. Sanchez's release will be completed quickly this weekend."
ELN chief negotiator Pablo Beltran said on Twitter that the force would provide instructions Saturday on how Sanchez will be released.
The ELN, like the FARC, took up arms against the government in 1964.
The many-sided ideological and territorial conflict has left 260,000 people dead, according to official data.
FARC militants are due to start disarming and demobilizing under UN supervision.
Their leader Rodrigo "Timochenko" Londono warned the government on Wednesday over delays to the process.
It was supposed to start on December 31 but has been postponed by a month. Londono said the government had not yet provided material for building the demobilization camps.
"If the government does not immediately set up the necessary infrastructure, it will be necessary to reconsider the date of the FARC's arrival" in the demobilization zones, he said on Twitter.
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