Isa, it would be recalled, was arrested last week as part of an ongoing investigation into $2bn, meant to buy military equipment to fight militant Islamist group Boko Haram, that had allegedly gone missing during the rule of ex-President Goodluck Jonathan.
Mr Isa was the first ally of Mr Buhari to be arrested over alleged corruption.
He was accused of receiving $850,000 from Mr Dasuki.
Isa has reportedly been released after promising to return the remaining $350,000 he allegedly received.
Isa has said he was given the money to buy a house for Dasuki, and was unaware that the money had come from government coffers.
Mr Dasuki has not commented on the allegation concerning Mr Isa, but has repeatedly denied any wrongdoing.
Dasuki, a close ally of the ex-president, is standing trial for allegedly diverting $68m of government money.He has pleaded not guilty to the charge.
Several leading opposition politicians are facing trial over the alleged fraud.