Wow, I am very surprised that the vet allowed the dog to go home with an impacted gland without treatment. And for a MONTH!? A questionable anal gland should not be viewed again a month later. Something like that could become abscessed/infected and/or rupture externally within days, let alone months. All of those things are very messy and uncomfortable. If the vet cannot express it (which generally isn't reccomended if the gland already looks like this), it is usually lanced through the skin, lanced through the duct, or antibiotics given.
Anal glands become impacted because they are not expressing on their own, either because there isn't pressure being applied to them, or because some sort of debris (sand, dirt, dried poop) is/was blocking the duct either externally or internally. Does this dog have regular, hard stools?
The first couple of times that Kirby had an impacted duct, my scalpel happy vet at the time told me that he needed to be surgically treated. We went through with it. Down the road, we decided to have an anal sacculectomy done (and I have a graphic pictorial somewhere on here of alllll the levels of anal sac disease), which is apparently an uncommon procedure, but the episodes were so uncomfortable for him, and it had to be done. When a new swelling came up, the new vet said that they couldn't be operated on in the condition that they were in, gave 5 days of antibiotics, and the swell went down and they expressed enough on their own that they were operable at the end of the round.
What is interesting to me, is that over the past month, the gland has not grown, or ruptured, and that the gland does not appear to be more bruised or swollen.