capital managment services.LP paid off A debt with A less then full amount waiting sinse jan. for letter of guarantee Buffalo New York

Capital manangement was collecting A debt for citi financial in the amount of 1269.47. They offered 2 payments of 380.84 in the mean time I was sent A bill by citi for 888.63 I paid anouther 120.00 then I paid anouther 380.84. I was informed by Capital every thing was paid in full I keep getting billed by citi called them they need A letter of guarantee even though they have letters every thing was paid. City informed me if they sell off amount of387.79 I have to pay it. I have been trying sinse Jan. to get this letter of guarantee Capital keeps saying its on its way. citi keeps billing me not sure what I can do next