I'm considering a PhD and asked a professor at my university. I suggested a topic to him that he accepted. I applied and he offered me a position but I didn't accept yet.
Later he changed my project's topic to one that suits his interest. I said I would like to do the original topic that I got accepted with. Then I asked more about the project and it turns out he works in collaboration with another university. They divided the work up between themselves, but what the other university researchers is a lot closer to my personal interest than what he works on.
I asked my professor their contact details, so that I could ask them if I could work with the other university. He explicitly prohibited me from talking to them. He told me I would give bad reputation to him if I contacted them.
I am extremely confused as to why he prevents me talking to people. Even a simply inquiry email is not allowed. So I contacted the other university anyway. They told me that they would be happy to see me but they don't want to poach students from their colleagues.
I keep asking my professor if I could do my original topic with the other university that works on that exact topic that I'd like to do. But he keeps telling me that I either do his topic or I should do my PhD somewhere else.
I am utterly confused now and have no idea what is going on. Can someone please explain?