On 2 October, Tango told Firestone it had not been able to pay US$0.3mln into escrow as a deposit for the deal.
The new deadline is 8 April subject to Tango raising the US$7.65mln balance and paying the cumulative cost of the BK11 deposit’s care and maintenance programme.
If the conditions of the new agreement are not satisfied or waived by 8 April, the disposal will not complete.
The firm also revealed the bulk sampling programme at the Oena project in South Africa had restarted.
The firm's first diamond tender sale results from Oena sold diamonds at an average of US$2,480 per carat and the highest price received was US$6,016 per carat and the average stone size was 3.28 carats.
Theodore Boshoff will manage and oversee the bulk-sampling programme.
He has over 11 years of diamond mining and processing experience of which five years were with De Beers Consolidated Mines at the Cullinan, Koffie Fontein and Venetia mines.
Oena is a 8,800 hectare mining right along the Orange River in a well-established alluvial diamond-mining province known to produce high quality and large sized diamonds.