The elderly man was sent by the children’s parents after they managed to raise 30m naira ($73,000; £53,000) by selling land and other possessions.
But they have been left feeling hopeless following his kidnapping.
The north of the country is in the midst of a wave of school abductions carried out by criminals for profit.
Ransoms are frequently paid, but this is a rare case where the person carrying the cash has been taken.
The kidnappers called up the school’s headteacher to say that the money delivered was not the agreed sum.
The 136 students were taken from an Islamic school in Tegina, Niger state, in late May.
Gunmen riding on motorcycles stormed the town and opened fire indiscriminately killing one person and injuring another.
As people fled, the attackers went to the school and seized the children.
The parents and school administrators negotiated with the criminals and agreed to pay the ransom. They sold part of the school’s land as well as other possessions.
Headteacher Malam Abubakar Alhassan told the BBC that six people were sent with the correct amount to meet the kidnappers near the forest where the children were being held.
When they arrived, the gunmen demanded that one of the group, an elderly man, follow them into the forest so that the cash could be counted.
But they later called to say the money was not sufficient.
“Parents are now resigned to fate. They say they can’t raise any more money. They are now relying on God,” Mr Alhassan told the BBC.
More than 1,000 students have been abducted from schools across northern Nigeria since December last year.
Hundreds of them are still in captivity. The authorities are being severely criticised for their failure to tackle the country’s widespread insecurity including the deepening kidnapping crisis.