Bad customer service

After I have opened a laybye in January 2017 with a deposit and 2 payments Ive paid a last instalment and received my goods on 20th June 2017.When I want to make a new laybye in October 2017 their system showed that my last payment was never received and a balance reflects next to my name so I have to pay the outstanding balance before a new laybye can be opened but in their file the assistant who assist me highlighted my name nd draw a line through my name that I ve taken out my laybe on the date that ive paid my final payment with her signature nd the date of 20th June .That is the date that the system shows a non payment . So now they told me that I have to prove with my receipt that I have pay the balance only then they can clear the balance. I have destroy my receipts because it was 4 months ago .But they too cannot find no prove of payment or till slips in their books which means that the cash money did not go into their till but i have received my goods and the assistant signed that she signed my laybye out .So why didnt this laybye did not expired until now because its now 8 months old ?Its unfair towards a customer to be held responsible for handling cash at the till ..and now insist on prove of payment .