IF you had an agreement, hopefully, on paper, stating the color and the job, then it should be the builder that takes all responsibility to make the change.
Have you closed on the home?
I feel confident, if he/she is any builder at all, they will make the mistake right.
Dark Walnut on oak is very dark so the process to go lighter and redder is going to be quite a challenge to say the least. You can make a lighter stain darker, BUT a darker stain lighter is another story.
This will require sanding, and I am sure detail sanding too.
Document ALL the stuff you have and hold him/her to the agreement.