I know that @DrasticDreamer is correct in that it varies by state. It also varies by circumstance. In some cases, the collection agency gets, say, 25% of what they collect while the creditor gets the rest, but sometimes the collection agency buys the debt outright and any and all agreements with the original creditor is out of the loop entirely.
If the original creditor is willing to talk to you then deal with them directly.
If you are stuck dealing with a collection agency, go up the chain of command; talk to their supervisor or supervisor’s supervisor. My last dealings with a collector (other than my wife) went badly until I went over the head of teh fuckbitchhellcunt collection agent I was dealing with. She was unwilling to accept any arrangement other than payment in full, but the supervisor was a nice guy who worked with us.
If they refuse a good-faith offer of payment and you can prove it then you are off the hook completely and they have to suck it. Get that proof and make sure that they are not violating the Fair Collection Act
Now for an excerpt from the FTC
What practices are off limits for debt collectors?
Harassment. Debt collectors may not harass, oppress, or abuse you or any third parties they contact. For example, they may not:
* use threats of violence or harm;
* publish a list of names of people who refuse to pay their debts (but they can give this information to the credit reporting companies);
* use obscene or profane language; or
* repeatedly use the phone to annoy someone.
False statements. Debt collectors may not lie when they are trying to collect a debt. For example, they may not:
* falsely claim that they are attorneys or government representatives;
* falsely claim that you have committed a crime;
* falsely represent that they operate or work for a credit reporting company;
* misrepresent the amount you owe;
* indicate that papers they send you are legal forms if they aren’t; or
* indicate that papers they send to you aren’t legal forms if they are.
Debt collectors also are prohibited from saying that:
* you will be arrested if you don’t pay your debt;
* they’ll seize, garnish, attach, or sell your property or wages unless they are permitted by law to take the action and intend to do so; or
* legal action will be taken against you, if doing so would be illegal or if they don’t intend to take the action.
Debt collectors may not:
* give false credit information about you to anyone, including a credit reporting company;
* send you anything that looks like an official document from a court or government agency if it isn’t; or
* use a false company name.
Unfair practices. Debt collectors may not engage in unfair practices when they try to collect a debt. For example, they may not:
* try to collect any interest, fee, or other charge on top of the amount you owe unless the contract that created your debt – or your state law – allows the charge;
* deposit a post-dated check early;
* take or threaten to take your property unless it can be done legally; or
* contact you by postcard.