Undermined by senior manager taking former direct on promotion

Hi,

Both myself and my direct achieved promotion and will move to new teams. Others are in the same position. There is a collective agreement that transition will be managed carefully to prevent individuals getting caught between old and new boss work requirements.My current and new manager have agreed to keep my move on hold whilst i complete my projects and more resource is found to take over.My directs promotion start date has already been confirmed. I will receive mine next week. I have found out from my direct that he is being assigned work already by his new boss who has not consulted me. This means his workload will come back on me. This will increase my workload and delay projects. This boss is senior to me on a different team and sees this as my problem. During transtion we both report to the same manager. I would appreciate any advice as the sense of fustration is difficult to deal with.