the rule is, if you say you are going to get back to someone, do it. if you agree to a made plan, stick to it. and if something comes up, just say. it really is not that hard to say that you changed your mind.
unacceptable alternatives to the rule include: lying; disappearing. even in the guise of being nice, neither are acceptable. lie to someone about about some trivial matter of opinion if you must (e.g.: no, you do not look bigger in that; yes, this cake is delicious). but for the love of pete (or whomever), in matters of fact, just say it. le sigh.
i guess i figured that by this point if someone wanted to say that they did not want to do something, they would say it. of course, i am obtuse.
how dare you complain to me