Landlord issued an incorrect move-out balance, corrected it at the time but now it’s in collections

Last June, we moved out of an apartment and initially received a notice of a balance of ~$300 USD for damages. The amount was based on damage that had already been recorded when we moved in, so after we discussed it with the office (in person, unfortunately), the amount was revised and we paid that in full, with a check statement and confirmation from the office that the balance is cleared. We do have record of a new final notice with the update balance, and a copy of a check matching that.

Last week, we (myself and my roommates at the time) received a notice from a debt collection agency matching the original amount pre-revision. Basically what seems to have happened is the landlord used a separate billing concierge company to handle the billing, and when we argued the original charges, they cleared it on their system but didn't contact the billing concierge to remove it.

Now the concierge has wiped their hands of it, they say they never got any notice from the landlord so they sold it to debt collection as per their policies. The landlord says they don't know why it happened because our balance due with them is zero, and are "checking" (been calling regularly for a week, still no update/progress).

I'm a little unsure what the next step for this is. It's such a small that amount that irritatingly enough I think getting lawyers involved will cost more than just paying it, even if we don't owe this balance to begin with.