It depends on exactly what the lease said, but it may be legal. For example, say that the lease specified that you had to pay $1,000 down, of which you paid $500 and applied the balance from the refund to the rest. If, however, there was in fact no refund, then you would have been left owing $500 more--since the agreement said you'd put $1,000 down, but at the end of the day, you only put $500 down (since there was no money from the refund), you'd still owe the other $500.You need to see exactly what  you were obligated to in the lease to see if the failure to get the extra $500 or so from the refund leaves you obligated for the rest. You are right in that the contract cannot be changed--but you need to make sure of what you actually had to do under the contract in order to determine your rights.