who will re-sign something unfavourable?

Yesterday I signed off the user requirements document for a project, then today, the vendor asked me to re-sign it.

Me: Why?

Him: Oh, because we made a change.

Me: What change?

Him: We brought the signed document back and our boss told us to add in a line.

Me: What line?

Him: That you will sign off the functional specs within a week of signing off the user requirements.

Me: No. I am not signing.

Him: But ...

Me: No.

Him: Our boss ...

Me: Just this week, trying to find a date to go through the functional specs with you yielded no dates. I will only agree to such a term if you can promise me that you have the full 5 days blocked out from your other projects, and we meet consecutively 5 days to clear this. We still have a lot of ground to cover.

In the end, he just said ok.