A scam or a forgery. St. Barnabas was real enough, but this is just another of the endless efforts to subvert Christianity with fake documents.
There are some genuinely old documents that simply are not parts of the Bible. But since Mohammed himself was a fake, and the Muslim religion cannot be reconciled with the Bible on the basis of the “historical” facts it invents as well as the ideas it proposes—the Allah-approved goodness of rape, murder, slavery, and oathbreaking, for instance—the idea that Barnabas would have prophesied Mohammed’s coming hundreds of years before his birth is simply unreal.
The pictures are not very distinct, but even there it looks like a fake to me.