I am in the middle of having a house built with a poured concrete foundation. I have paid for two 30 50 windows in the basement to meet code if I ever wanted to place a bedroom in the basement. The other day my builder told me that the person who poured the basement made a mistake and I now have two openings for 50 30 windows, which the base of the window is too high for code. They offered to cut the foundation down to length and fill in the two foot width. My question is this often done? Is it advisable or should I just leave it alone. I not sure what my options are.

Well, your builder know's best. So if you trust him, then trust his advise. But, if code req. the windows to be a certain size then I would advise they be done. Myabe ask what kind of warranty he may give on the foundation work? Ask if it will weaken the foundation by changing it or is it best to leave alone.

Does repair suggest a less satisfactory job than fix? No substitute for doing the job right in the first place? Wonder if they got the foundation drains and barriers right? Better to do it now than later.

If you have them correct it, make sure they drill and install rebar into the existing foundation so the patch is "locked" in. What kind of egress window well system is being installed? May want to check code on that also. Looks like grade will be even higher after the opening is cut.
The mistake was made, hope you don't have to pay for the correction. I would have it done because in the long run you will say that you should have. They people, tools and machines are already there.