That is the point...the principal will investigate if this situation is breaking the rules. Why is it ok to let a teacher break rules (if that is the case here) whether it concerns me or not? Think about it!
1) It doesn't concern you.
2) It is your responsibility to make sure the teacher is actually breaking any rules before reporting him/her.
To make false reports without facts is so irresponsible I just can't wrap my head around it.
If you are afraid of conflict, then have your husband or a fellow parent ask the teacher.
There is nothing wrong with going up to teacher and saying "hey, there are rumors going around the grapevine that so-and-so's parents are giving you some tickets in front of the classroom and some parents are getting resentful. You know how rumors can be blown out of proportion, so could you tell us what is really going on?"
IF you find out that he/she is accepting gifts against school rules, then, and only then, should you go to the principal.
You never answered how you would feel if a fellow employee accused your husband of something based on rumor to his boss's boss without checking with your husband first if it was true.
What if it was the fellow employee's wife that contacted the boss - as you know, since it did not involve her husband, she didn't need to check it out first?
If your husband's boss's boss had to check out the rumors, do you still think it would be innocent when your husband has done no wrong? Or would your husband have been insulted and humiliated by being accused of such a thing? Would your innocent husband have been angry that this person, who wasn't even involved, went over his head, accusing him of something without verifying facts first?