The people paid $30 originally, with the $5 taken from the till that makes
the actual cost $25. The people paid $27 less the $2 the clerk took, it
becomes $25.

Very close. In fact, you may have it figured out and I'm not quite bright
enough to follow your explanation. The problem confounds some people who are
led into doing a certain type of math.

The $9 apiece is really a red herring. Like good propaganda, it uses someting
that appears logical to support something that is not true.

$30 aggregate from the buyers. At this point the cost of the goods was $30.
That
changed to $25, momentarily, when the manager instructed the clerk to refund $5
fom the till. The cost went back up to $27 when the clerk "reduced the refund"
by two bucks. Add the dollar apiece given to
the three posters to $27, and all $30 is accounted for.

But 3 x $9 is still $27, and the clerk took $2.

Amazing what you can do with a carefully selected portion of the truth and a
deliberately calculated presentation.