MMU and settlements

1st car i bought from MMU no issues, traded the car back to them, 14 April 2024, they never settled and I paid installment for 30 April 2024, which I was not supposed to, the traded car was only settled about 6 months later, all other installments were paid from May until they eventually settled the car, I am owed an installment i was not supposed to pay, I tried all avenues MMU even evades lawyers