First national collection burea Fncb bank Made an agreement I borrowed money to pay it off now they want all the money sparks, Nevada

I made an agreement with Fncb to pay off a debt they told me the original debt was 445.85 for a 300 credit card that i only used half the balance they offered me a seettlement of 50 percent off I made two payments then borrowed money to pay it off because I am trying to fix my credit.  after getting a paper with a o balance printed out.  I recieved a new letter in the mail telling me I have only paid half and the rest is due.  

There has to be some way to handle this i have paperwork that says my balance is paid in full.  Today i get a new letter saying only half the balance is paid.  This has reall made me consider not paying off any more debt.  This has truly left a bad taste in my mouth for trying to do the right thing.  If this is not resolved i will spend money to promote this crap online so nobody pays of fncb ever again.