Did the shop do the set up/assembly of the bike after they built it? If not did they ask for the specs that you were planning to use (like rims, brakes)? Seems to me there's more to the story yet to be made known.

One aspect of U brakes, unlike cantis and Vs, is that since the brake arms need to have a interlacing relationship that places limits of the pad mounting surfaces' width apart. As in minimum and maximum across the pad mounting faces or eye bolts. This in turn will influence the rim width a particular boss/brake model/pad combo will work for. So having all these elements known and on hand before any brazing (or for that matter any acceptance of the combo) is vital to avoid what the OP is suggesting happened.

Again, what's the entire story? How much detail was shared? Did the framebuilder provide the parts or hand the customer provided parts on hand? This is a sticky wicket with a lot at stake on both sides. Andy.