If we define sin as 'to fall short of the glory of God,' then I suppose we could say that you have sinned if you break your fast before you had intended to. Furthermore, Psalms 15:4 says that God honors the man who 'swears to his own hurt and does not change.' And Ecclesiastes 5:5 says that it is 'better not to vow than to vow and not pay.'
These verses are telling us that when we say either to God or another person that we will fast for a particular period of time (make a vow) and then break the fast before that time is up, we have failed to keep our word. We have said we will do something and then not done it. The sin is not so much the premature breaking of the fast; it is the failure to keep your word. Jesus said, 'But let your 'Yes' be 'Yes,' and your 'No,' 'No.'' (Matthew 5:37). In other words, mean what you say. If you say you will do something, then do it.
Having said this, we believe it is also important to realize that we often learn alot about ourselves through our failures. When we intend to fast for three days but break it half way through, we come face to face with the stark reality of our own human weakness. Rather than condemning ourselves as hopeless failures, we need to take our failures to God and ask for forgiveness and strength.
In Psalms 35:13 David said, 'I humbled myself with fasting.' Sometimes our failure to fast can be humbling as well. This is not to excuse the failure to keep your word or a lack of self-control. But it is to say that the ultimate purpose of fasting is to help us to become more cognizant of our weak humanity so that we might be more humble before the Lord and more dependent on His power. This can come by fasting just as we intend, but it can also come by our failure to fast as we intend. God can work ALL THINGS together for good!