When the UK bought Rupert's Land and gave it to Canada it STILL wasn't purchasing THE LAND, only the sovereign authority the HBC had.
From Wiki:
"The Rupert's Land Act 1868[1] (31 & 32 Vict. c.105) was an
Act of the
Parliament of the
United Kingdom of Great Britain and Ireland (as it then was), authorizing the transfer of
Rupert's Land from the control of the
Hudson's Bay Company to the Dominion of
Canada. This is the largest land purchase in Canada's history. The transfer occurred in 1869 and was consummated in 1870 by the payment of a consideration of
£300,000 (£27 million in 2010)
[2] to the Hudson's Bay Company, as mandated by the
Rupert's Land and North-Western Territory Order of 1870. Under this agreement, the Bay Company also retained rights to 20% of the arable land in the territory. In short, this was an act giving Rupert's Land to Canada."
Rupert's Land was not an independent nation. It was part of the British Empire. Britain gave HBC full ownership of the land and authority to administer it, but it did not give it independent status.
Of course Britain had responsibilities toward Rupert's Land. It was part of the Empire - a possession of Britain the same way my land is a possession of the US. I don't have quite the same amount of leeway to administer my land as HBC did, but the concept is the same. It's doubtful Britain would sell the possession, but they could if they wanted to, unless there was a clause in the charter.