Riverstone Residential Group Greystar Put a collection on my credit report after paying in full Lynnwood Washington

I was sent a final invoice for an apartment lease that I had. The invoice said I owed $1155 and that would pay my account in full. About a week later I was told that the amount to pay in full was not accurate and that I owed approx $7300 more. I had a $1150 deposit when I moved out that was in there account and asked for an accounting to see how I owed so much more. After negotiating the amount owed i agreed to pay them in full with two payments. I was told that as long as I paid the first payment the day we agreed nothing would go on my credit. They then sent the credit bureau a collection notice that I has an unpaid account for over $3700. When I called to try and resolve this I was told to send them a copy of the letter from Greystar that my account was paid in full. I then asked if my account was paid in full why would it show unpaid on my credit report?  She told me there was nothing she could do and that was it. I waited over a month for the accounting from  Greystar to get the actual balance I owed and after agreeing to pay in full was still sent to collections. The only one who can take this off my credit report is Greysatr and they refuse to do it after doing everything that they wanted. This is not how to treat people who are trying to do the right and ethical  things. Obviously they had no intention of honoring what I was told on the phone where there is no record!