I live in Washington state and purchased a used vehicle last month. The used car dealership & I sign a contract which is already sold to a local credit union. On the contact I made a $2100 deposit. 500 was on a check, 1000 was cash and 600 was on the debit card. Now the used car dealership gave me a letter that i still owe them 1000 on the down payment and wants proof of payment. How can i prove I paid them in cash?