Did you wet the peat moss first? If not its probably still dry.

Yellowing leaves usually indicate too much water. Are green leaves drooping or dropping also? I just lost a 10 year old plum tree that was happily planted and growing well. This year it did not bloom but still looked good. Now, in the last two weeks, it dropped 1/3 of the leaves on one side, then the center drooped. It has been diagnosed as having an internal black rot disease. I guess it spread very quickly. I had to chop it down.

For the soil, I would have added Pearlite or volcanic glass.

Did you unwrap the roots? Is it root bound? It may need a larger pot. Eventually it would cause girdling and strangle the plant. However, you suggest that you have had this but a few weeks and that would not happen this soon.

You probably go back to the seller and ask. While you are there, look around to see if other plants show distress.

I hope something in there helps...

Need a picture of the entire tree, please.