I think that's a big problem with some of the PFD sample letters that are floating around. They start of by disputing the debt.
A lot of CA's seem to not read past the first paragraph. They see the dispute, they respond by sending validation.
So just send another PFD. Include something that says you aren't admitting you owe the debt, but you are willing to pay it to get it off your report. Keep it short and simple and put the most important part in the first sentence.