You also need to step back and understand that when they say,QuoteShould we release this matter back to Masterfile, I assure you, they will no longer be willing to settle for an amount anywhere close to what they are prepared to accept at this time, it sounds very threatening and scary to you.
What it really means is, "If you won't pay us, we are powerless to make you pay us, and we are going to give this back to Masterfile as uncollectable." It doesn't mean that Masterfile is going to do anything. At best, they might try to give it to another bill collector, but the odds of their filing suit over an amount this small . . . you should be able to figure it out.
While I mostly agree with this, one other thing that should be considered, is that history tells us MF does file suit more often, and they also seem to have better registrations in place.. Truth is , if the OP doesn't have the stomach or will to continue with the back and forth, it might be best to simply hire Oscar and let him handle it.