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FYI, be careful with apartment complexes - they are notorius for getting a judgment for uncollected amounts. Proceed with your approach to negotiate a PFD, that is the most effective way to handle this - otherwise it turns into a much larger problem. I regularly see other people's credit reports for rentals and this is one of those areas of collection where the apartment complex (especially if they are one of the mega complexes) goes after people like a pit bull.
If they will do a PFD for an additional $50, I would jump on it if removal of the collection is a primary goal.
As for sending a DV, it is likely too late.
A DV must be sent within 30 days of receipt of their initial collection ("dunning") notice to be timely.
However, even if timely, a DV does not set any period within which they must send validation, and does not require any response.
A timely DV imposes a cease collection bar, and as long as the debt collector does not continue collection activities, they can choose not to send any validation.
You are thus in a state of limbo......