Knowlton and another bidder, a Las Vegas businessman named Michael Luzich who paid $200,000 for his chance to hunt a rhino in 2013, will be able to access the permits when they verify that their money has been paid to the Namibian government.
This is the second and third time that the agency has allowed black rhino hunting trophies in three decades.
The government agency defended its decision by arguing that old rhinoceros bulls can prevent young animals from mating, and that allowing foreigners to trophy hunt helps fund local conservation efforts.
Shortly after the announcement by USFWS, the animal rights group PETA announced that it will file a lawsuit over the decision on Thursday.
"These permits are fundamentally inconsistent with the purpose of the Endangered Species Act, which is to conserve endangered species, not to authorize their slaughter," PETA Foundation deputy general counsel Delcianna Winders said in a statement.
In the early 1960s, there were some 70,000 black rhinos in Africa. Now, thanks to relentless slaughter for their valuable horns, there are some 5,000 left - 1,800 of them in Namibia. Sold as traditional medicine, rhino horn can rake in $45,000 per pound, often on black markets in Asia where it is believed to cure cancer.