I purchased furniture from XXXX XXXX XXXX in XXXX. I agreed to pay $ XXXX thru Synchrony bank for the cost of my purchase. I was told that the due date is the XXXX of each month. I have XXXX XXXX  records that I've paid as agreed. 

I was NOT told that a payment made prior to the XXXX of each month would be credited as a payment for the PREVIOUS month. XXXX is claiming that I have past due balances because my EARLY payments were credited as payments for the previous month. 

In a phone call between XXXX, myself, and a female in the XXXX accounting dept., the XXXX person explained why their system shows an erroneous past due balance. She said XXXX would erase/cancel the past due claims and advised me to just be sure I paid AFTER the XXXX of each month. 
I thought the issue was resolved, but XXXX is STILL making collection calls about this ERRONEOUS past due balance. My credit score has taken a big hit, and XXXX is the reason why. Please help.