What is the ruling on saying "Allahu Akbar" with an open ra in the takbirat of the prayer?

The diacritical mark for the letter "ra" in "Akbar" is a damma, because it is a nominative predicate (khabar marfu'). When pausing on it, the "ra" becomes silent. The Imam's opening of the "ra" is a grammatical error that does not invalidate the prayer, because a linguistic error (lahn) in recitation does not harm if it does not change the meaning. The Imam should be alerted to this.