A team of AEC nabbed Pankaj Gangar, who is involved in a betting racket and provided funds for Kaskar's extortion activities.
He was picked up from his residence at Sodawala Lane in Borivali west, north-west Mumbai.
Earlier, Kaskar and two of his associates - Israr Jamil Sayyed and Mumtaz Ejaz Shaikh - were picked up late on September 18 for threatening a prominent realtor of Thane and since 2013 extracting four flats in a posh area and three million rupees from him.
Police are on the lookout for some hitmen from Bihar and Mumbai as well as the person or persons who supplied them with weapons to threaten builders and extort money from them.
On Wednesday, a court here extended by four days the police custody of fugitive don Dawood Ibrahim's brother Iqbal Kaskar and his two associates.
The Enforcement Directorate has also begun an investigation against Iqbal Kaskar, and three others for extortion and illegal transfer of property.