It's plausible, although they wouldn't have used that phrase. European coin-makers sometimes copied Arabic coinage designs without paying strict attention to the religious content of the text. Here's one example of a coin from Britain. In the center you can read "Offa Rex" while the reverse is the Arabic shahada "La ilah illa Allah, Muhammad rasul Allah" "There is no god but God, Muhammad is the messenger of God," albeit it somewhat garbled at the end. https://i.pinimg.com/originals/12/0d/42/120d422e79384da7fb6442b4e7057c7e.jpg

There are other such examples of either outright copies or new designs that borrowed Arabic elements: http://www.caitlingreen.org/2015/03/some-imitation-islamic-coins.html