"Let your yes be yes is good advice domestically as well as business wise. In short be true to your word and if you break it (your word, your vow, your promise) you have lost honor in the eyes of those you have vowed to.
We certainly don't want to break a promise (our word) to the little ones in our lives... even though we know that they will be quick to forgive.
However in business if we break our word the affected party(s) may not be so kind to forgive us. Some Realtors seem to forget that breaking our word in the public realm can greatly effect our business relm (circle) as well.
Did you join a church or a club to help you enlarge your Sphere Of Influence (SOI)? Did you volunteer to help your child's after school sports endeavors? Only to find later that you "just didn't have the time".
Breaking these kinds of promises are just as negative as breaking a business agreement.
The only difference is that now, instead of upsetting one person you have set up a large chain of people who may be thinking "he/she didn't follow through on this, do I really want him/here handling one of the biggest deals of my lifetime?".
Let your yes be YES!
It may be even more important outside of the business world as it is in the business world. Think about it.