Dust from the Sahara blows across over to the Amazon, enough of the stuff makes it across to be a factor when it comes to nutrient supply for the rainforest.

The second one. The sands of the Sahara are vital to the Amazon rainforest. They blow all the way to the Amazon and deposit phosphorus which would totally get washed away without that deposit. The surface of the earth is hyper connected. You only experience yourself as a separate being from the rest of nature just because of the limited awareness we have, not because we are actually independent.

Apart from what others have commented on, some of the sand gets deposited in the ocean when it blows across the Atlantic. That sand is eaten by sea life and goes back into that ecosystem. The world is amazing.

Overall I think making the Sahara more green would be a net positive, but I'm sure there are unforeseen negative consequences for parts of the world.