Don't be scared just because the office threat to give it to the collection.
If they really do so and the collection contact you, you say you are
disputing the charge and they cannot do anything further.
First, how did the office mention the amount you owe them? If in mail, as I
said, mail back, dispute and refuse to pay until they provide details.
Without invoice to support or predetermined amount, something already
provided to you before lease termination, like how much standard charge for
different damage, they don't have the basis to charge you. As long as you
response timely, the case is open and I don't think they can report to
credit agency.