What if vow taker declines the term while taking the Unbreakable Vow?

So a very straight forward question. We know very well that breaking the unbreakable vow leads to one's death but here I've gone just a bit specific. My question is,

What would happen if the vow taker declines to accept the term while taking the vow?

It would be much better if you submit any reliable source in the answer. Anyway I didn't find anything related to this even on Pottermore.