I don't believe you, I think you are guilty of theft, and I'll tell you why:
You say "I gave my copy of the invoice, showing the deposit paid and the amount outstanding...."
Let me put this scenario to you:
Mr Bloggs pays a deposit for an item, goes back to the store at a later date, gives invoice to staff who then return invoice to Mr Bloggs before going to the warehouse to get the rest of the goods. Mr Bloggs realises that the staff haven't asked him for the balance outstanding, and keeps "shtum" even although two staff members spend ten minutes helping Bloggs to load the goods into his car, after which they say "cheerio Mr Bloggs" and return to the store. Bloggs, realising they haven't asked him about the balance, can't believe his "luck", gets into his car and drives off. He later has pangs of guilt and decides to seek further advice about his misdemeanour.
Ring any bells?