The Moscow City Court decided to re-examine the prosecution's case against Mr Bakhayev.
Mr Dadayev confessed soon after his arrest, but later alleged that he had been tortured into making the statement.
Investigators say they are still trying to determine who ordered the killing, which happened at night on a bridge near the Kremlin, in the heart of Moscow.
Friends of Mr Nemtsov say he was targeted for his political views — as a fierce critic of President Vladimir Putin and of the war in eastern Ukraine, where pro-Russian rebels are backed by Russian "volunteer" soldiers.