While I can't speak to that specific image, I can confirm that it definitely did take place. From Canada, thousands of children were adopted for substantial fees. In this CBC article is a report on specifically hundreds of Manitoba First Nations children who were adopted to American families, sometimes for as high as $30,000, during what is known as the 60s scoop. This was a period of time during which throughout Canada and the United States, various child/family service organizations had policies which tended to unfairly target First Nations/Native American children for removal from their homes and communities. An incredibly high percentage of children were apprehended, with some reserves reporting no children younger than 18 present.

This state of affairs in Canada lasted from the 50s into the 80s, with some 20,000 children taken from their families and adopted into mostly white middle-class families.

In my work doing language revitalization I encounter these people weekly as they make an effort to reconnect with their culture and communities. This is an example of one of the long-term impacts of these practices, as adults living as far afield as Australia, England, the USA, try to find a sense of belonging.

Going back to the specific letter you linked, I don't doubt that the letter is legal, but it looks more like somebody making a donation to a boarding school, possibly in exchange for having some kids come and stay with them. While it might have been connected to some adoption, this seems to be something other than adoption, and not a legal document.

As to how these things were perceived, they were government policy enacted for the public good, and your attitude likely depends on who you are. If you listen to the radiolab episode adoptive couple vs. baby girl you can hear a range of different attitudes expressed by different affected families in the American equivalent context. My favourite quote is something like "we all thought this was our own private nightmare until we started talking to other nations and realized it was happening to all of us." It was a big enough issue that it resulted in pretty serious legislation in both Canada and the USA, I think the US was the Indian child Welfare Act, and in Canada, I don't know the name of the legislation (I think it might actually be a range of changes to various bodies of legislation), however it is very similar in that the nations have a lot more control over fostering, adopting, placement of children, and so on.

This continues to be an issue to be honest, as there are still an incredibly high percentage of First Nations kids in custody, and a lot of serious issues continue to appear regularly in the news, for example APTN reporting on 102 deaths of Aboriginal kids in foster care, and similar situations, investigations and conflicts are still a regular occurrence across the country.