If we want them to stop developing the rainforest for economic reasons, then we need to give them an alternative. The fact is, the whole world needs the Amazon as a carbon sink. It seems like a worthy trade to me. They're helping all of us, and if they ever stop, the rest of the world can just cut the funding.
It's not like the rich countries are self-evidently sober and stable in their politics and climate impacts. The richest one just had their own wannabe fascist and has both been responsible for a large part of emissions and rarely met their climate goals.
There are many arguments to be made here but this isn't one. The money can have conditions attached to it. For example, give an amount now and agree to some target being reached within 6 months and so on.