I presume that by stating that no letters have been sent to you, you are stating that they did not send dunning notice?
If they have not sent dunning notice, then any request you send for debt validatiion will be timely, and its effect will be to impose a cease collection bar on the debt collector, which will remain in effect until such time as they choose to provide the requested debt validation.
They are under no period for or requirment to send validation. They can choose to simply cease additional collection activites until they decide to send validation.
If they are under a timely DV, they cannot conduct any collection activities, which includes any settlement or PFD negotiations.
Thus, if you wish to make a current PFD offer, you should not place them under a cease collection bar.
Howver, if you contest the debt, then it is usually more important to pursue validation before conceding to payment of the debt.
