Salon St.Jacques whole sale merchant hired a creditor and is coming after me for 4,000 for a machine that i sent back to them eugene, Oregon

whole sale merchant prossing i served my three years cancled them after that with a note they said  to send  sent the machine i used back they said they never recived any note or machine .the machine was lost in mail I had no proof that i sent it it had a tracking order never found .I realized i send the wrong one any how tried to send the right one they said it to late it went  collectoins .they dont want this machine that is worth well you know you can buy a brand new one for 150.00 they are after me for breeching cotract and a visa machine and pin pad that i have i my possesion . they dint want back  for around 4 thousand dollars.