Won't the cast iron underneath the glaze start to rust if I don't do something about this?

How terrible! You must be so upset about the scratch.
You could demand a discount if you do not want to
have it replaced. Or simple insist they replace it.

I have seen this site about Kohler sink repairs.
Not sure if it can help your sink or not.

http://www.surface-repair.com/nick-fix-enamel-porcelain-steel-touchup.htm

http://www.surface-repair.com/diy-kohler.htm

{{!gwi}}

Perfect for small chips & scratches this touch up works as a filler and repair glaze all in one.
Use on cast iron, enameled steel, vitreous china, porcelain, gel-coated fiberglass, acrylic, and more.

Thank you, boxerpups. I'm going to try that if I can't get the builder to do anything to fix this. Does anyone have an opinion on how I should approach the builder on this issue? I'm almost certain the plumber did this when he installed the faucet. The sink was fine before the faucet went in and no one else was working in the house after that. Of course it can't be proved but it looks as if a belt buckle or a tool belt was repeatedly bumped or rubbed against the front of the sink as the mark looks like a smiley face. I'm certainly afraid of any attempt to replace the sink since the cure will probably be worse than the problem I have now. Should I ask for some sort of monetary back charge against the plumber? Is there anything else I could ask for as a kind of make up for having to live with this? Although maybe in time I could think of this as "patina" right now it is downright ugly.

flseadog pointing to scratch, "Mr, Builder, how do you plan to take care of this problem?"

If he proposes anything that is unacceptable to you, say so.

Personally, I would insist on a new sink. It will come out of the plumber's pocket or his insurance company's pocket, if he has coverage.

Sounds like a belt buckle to me! Stand your ground! I know you feel so sick about this believe me! The only thing that ever worked for us is holding back money. Unfortunately the builder needs to work it out with his/her subs. They/He doesn't get paid until all your issues are resolved!!!!
I personally would want another sink. Easier said than done I know. We are 6 months into our house and I agreed to keep the wrong sink in order to forego hunting down another slab of granite.
Good Luck and don't back down until you feel good. Otherwise you will always regret it, AND you will get more mad later, especially after moving in.
:)

I understand how a belt buckle could scratch the front of the sink, but how the heck can it dent it? I would imagine that the discolouration can be cleaned off and you will be left with a perfect surface again. If it did indeed dent from a belt buckle then that plumber is superman.

Thank you, everyone. We are meeting with the builder tomorrow and I am going to do that "point and ask." I want to hear their suggestions and remedies. My biggest problem now is that I doubt I can get another granite slab to replace this countertop if anything happens while trying to put in a new sink. We picked 2 consecutive numbered slabs for the kitchen after we looked at all the others in the lot. The 2 we picked were very well matched in color tone and movement but the others all looked a bit off from what we wanted.

Advertguy, it isn't a dent, but a scratch in the glazing and I can feel it by running my fingernails across the surface. I will have to say that the plumber was definitely not superman but had a super big gut. The picture I have is of him leaning across the sink to adjust the pull out on the faucet and then swiveling to reach a tool. It's putting a shiver up my spine just thinking about it.