CR is the bext approach. As is discussed in minute detail through this thread, this was a safety issue and pax could have been IDB with no comp. The GA screwed up and likely realized that s/he screwed up and that's why pax didn't get the voucher.

By writing a short note, devoid of emotion which does not infer facts OP does not know, CS might issue voucher or something close to it as a matter of CSR. The short note ought to be 1/3 the length of the post and simply focus on fact that comp was promised and not given.

As to disputing the charge, even if this were not an award ticket, it's foolish and risks having the issuing bank cancel the credit card for a bad faith challenge. The CoC was honored here and one can't dispute a charge because another matter was or was not handled properly. Even if this were a paid ticket, you can only dispute the charge for the ticket, not whether the same vendor owes you money for something else.