I really doubt he is a Russian asset. He is just doing exactly what he said he would do in recent speeches - putting the USA first (and not giving a damn about anyone else in the process).
He sees an opportunity for the USA to make a lot of money by getting mineral rights in Ukraine for no cost. He is quite prepaerd to bully Ukraine into accepting whatever deal is agreed and to cosy up to Putin at the same time to achieve it. He doesn't give a damn about Ukraine, only the money which is to be made by mining their land and he has seized an opportunity to get it.