I would never commit to something if I wasn't decided. Just like when I was offered a job with another company last year, when offered, I told them I would make my decision after speaking to my current employer.
After he gave me his offer to keep me, I went back and said unfortunately I wouldn't be able to take the job unless they agreed to X. They didn't, and we parted ways.
What I didn't do, was agree. I then didn't go an tell everyone I was 100% going to go to the new job, I didn't continually lie and say that I was going and that nothing had changed. The one time they called me through the week, I advised them I was not yet decided.