Landlord insists on collecting additional 3 months rent despite contract not stating so

[US-MI]
Hi All, I need some advice. We gave notice of moving out on Dec 17 and handed the keys to the property manager on Dec 31. I sent out multiple emails to the property management that keep insisting we owe them additional 3 months rent despite the fact that our contract states:
“IF Tenants decide to break the lease before the lease contract expires, Tenants agree to pay $1500, in addition to all the rent due by the time they move out.****” -- pic here: https://imgur.com/a/wHN2FO5

I've already sent them a check for $700 along with a letter explaining that we only owe them $1500 minus $800 security deposit yet they never reply to either the letter or the emails. I had sent them an email on Dec 26, Dec 29 and they never responded. Now they've sent my account to a debt collector and I really don't want my credit score to take a hit - what are my options?

Am I incorrect in my understanding that I do not owe them any rent since I'd paid all the rent due by the time we moved out?