No, I would venture to say that that would not be a legal deduction from an employees paycheck and doing so could get the employer in some trouble.  Your employer can bill you for the non-attendance but he would have to prove that you said that you were attending.  I would speak with your boss and explain the situation in a very calm and well thought out manner.  That you did not RSVP to the event because you did not intend on going because - give a good reason here: other obligations or whatever the reason was - and that you are sorry for the mix up, however that occurred.  And that you do not believe that that type of deduction is permitted from your paycheck.  Then see what happens.  You may have to report him to the department of labor.  Good luck.