Does it do any good to call a Collection agency

What you should know is that most collection agencies have NOTHING of the sort in their files.
Actually sending the CA the billing records would cost the original creditor money time and effort. Since 99.9% of alleged debtors do not request validation, the vast majority of debts, especially ones for less than the price of a new car, do NOT come with this information when sold to the CA.
The CAs typically have some identifying information about you, as well as the amount of the debt. They sometimes have more information, but frequently not much more.
A couple of years back I was trying to pay off a medical debt and I’ll be danged if I could even figure out WHAT the bill was for, WHEN the alleged service was provided and WHO performed the alleged service. Even the lady at the CA who was trying to be helpful so they could get paid was unable to provide us with this information. They didn’t have it.
And I’ll second getting a lawyer, especially if this is worth suing over.

The first sentence is pretty much accurate.
I’ve been tempted to name a law firm in India or Japan as my representative just to muddy the waters, but don’t figure it would be worth annoying the law firm.
“Yeah… ummm… my lawyer… don’t have his name handy… he’s out of New York… I think the last name’s Cohen. Just look him up.”

Your second sentence is probably not accurate.

From a purely legal standpoint, you could be held liable for some (the difference in cost between the first unit and the current unit) or all of the bill. You are getting benefit from the unit at this time, that will work against you. I was in a small claims court a few weeks ago with a friend and watched a similar case, it involved an A/C unit, not a heat pump. The homeowner claimed she never wanted the unit, it was too expensive. The contractor said he could not get the unit the woman wanted but he had a more expensive unit. He then said she told him she would pay anything to have A/C. He showed up and installed the more expensive unit, she thought she was getting the cheaper one. The judge split the difference, the woman would pay the cost difference between the two units (about $100). Neither seemed real happy about the decision.

Definitely talk to an attorney though, there may be some consumer protection laws to help you.