letter demanding 100 pct in 10 days

I got a letter demanding payment or litegation for the second time from a creditor. I sent them 3 settlement offers for up to 50 percent, all rejected. Then sent in a payment of 179 to buy more time. They would not discuss setteling for less than 90 percent. I told them I did not have that and talk to them next month. The next person that called said they charge off after 180 days. I'm at 80 days after sending the payment. I settled with Citi, but had to wait until it went to collections for 41 percent. This last creditor is Key Bank. They just sent me the second letter that is the same as the letter last month saying pay in full in 10 days. Please advise on what to expect next and how to get them to settle lower percent.