Singh, who was questioned for three hours on Friday, can be summoned again by the Delhi crime branch over the same scam.
Singh has been indicted in the scam for allegedly sending money as bribe to win over three BJP lawmakers during the 2008 trust vote in the Parliament. The Home Ministry earlier had issued the go-ahead to question Amar Singh.
Earlier, Hindustani, who was an aide of Singh, had named Sonia Gandhi and Prime Minister, Manmohan Singh in the cash-for-votes scam.
"I received calls from Amar Singh and Ahmed Patel (Political Secretary to Congress President Sonia Gandhi). Those close to Prime Minister (Manmohan Singh) called me and even from 10-Janpath (Gandhi's residence)," Hindustani had said before police arrested him on Jul 20.