While the lump is the most obvious problem. it isn't the most pressing. First off, I'm gonna ask a few questions to get a slightly better idea of what's going on. Is your vet rabbit-savvy? Are you sure she hasn't pooped at all in 4-5 days? Not pooping for 24 hours is an emergency in rabbits, so the vet should be extremely concerned by that information. Did you tell the vet that she hadn't pooped in that long? Is she showing evidence of being dehydrated or being in pain (grinding teeth, hunched position)? Is she behaving differently? Has she pooped since?
A lump is not necessarily something that needs to be removed, depending on what it is and where it is located. It could be a cyst, a benign tumor, or unfortunately a malignant tumor. None of these things will kill a rabbit overnight, but GI stasis (the digestive system stopping) will kill a rabbit extremely quickly. A rabbit can live a full life with a benign tumor and surgery is not always necessary in that case.
To sum up, I'm far more concerned by your rabbit not pooping than the mass. The mass can be solved in time. What you are describing is GI stasis and is an absolute emergency. If your vet wasn't worried by this, you need to find a new vet ASAP and go to them immediately.