@jeyps11_c, you should really speak with the manager, and if that doesn't work go up one level higher, until you reach the owner. Since you are getting the receiver in a worse condition, Lotteworld should take responsibility for that.
If they insist that the other inputs (FL/FR speakers, coaxial, etc.) were not working in the first place, then it's going to be tougher for you. They might resort to this tactic - but hopefully not. If you have a receipt of the job order stating that only the optical 1 was not working, this will help your case.
Regardless, you should talk in a professional manner to the supervisor first. If he cannot help you, demand to speak with the next higher person. Usually, there is always someone up the chain who is reasonable. In case you exhaust all possibilities, e.g. even the owner won't listen to you, the next step would probably be to file a complaint with DTI. Or perhaps a lawsuit. You may want to get a lawyer if it gets to this point, but if you feel very strongly about the injustice, then hiring a lawyer is worth it.
Another option is to take the unit to a 3rd party repair center / technician that has a good reputation. Unfortunately, I do not know of any such repair center or technician. Maybe other PDVD members could help out.
I also have a Denon (1610) so I thank you for sharing your experience. It might come in handy in case my unit requires repairs later. Good luck to you.