This lens has a fluorine coating on the front element which can be susceptible to this type of damage. There was a post a year or two ago here where someone messed up their 600/4 in a similar manner. You'd have to test it to determine if it results in any visible image quality degradation. Probably not in 'normal' conditions, but uncertain if, for example, strong backlighting could cause additional flare.
I believe the only fix is to replace the front element, especially if the intent is to resell it (unless you want to take the hit on the resale).
