First of, Manna already got a value. Every coin can as I write this be traded for $0.003055.

From your question it looks like it's gone be hard to change your mind on what money is. It's seams like you think there is some underlying value to currencies. It is not. It is just pretty paper printed in the rate the central banks feel like. The value lies in the network of people trusting it and using it for trade. Usually every single person in a community/nation use the same currency, and don't question its value.

But British Pound is great for trading in Britain, but in Germany you wan't be able to buy anything with them. There is no network or trust for Pounds in Germany.

If you just want to keep asking skeptical questions, don't. I don't feel like putting in a lot of effort to answer, if you not gone accept any of my answers anyway. But if you truly want to know about the value if currencies, start by watching the video above, and then do a YouTube search on "fiat currency", and start consuming. At one point you should start asking the same questions that you ask about Manna, to the currency you have been using your hole life.