Cancelling contract

We started work on a project yesterday and completed about $3000 worth of work when we had only received a $1500 retainer deposit. Our contract terms require a larger deposit on day work starts and we made 3 attempts yesterday to collect the deposit, all of which were ignored. Can we release the client from the contract and move on? I am not trying to collect anything, we are just too busy to track down money and since this is the first payment, it seems like a sin of things to come. I know the answer seems obvious, but I want to make sure I am protecting myself legally and that we can email a release from the contract and go on to our next project without repercissions. Thank you