How do I write a letter to void a mortgage contract based on finances?

Once you have a valid mortgage, and it is signed and in place, you can not just void you, you are obligated to the terms of the contract.

Can you please explain more what you want to do. Do you have a mortgage and can't afford to pay for it,

I assume that you are seeking to void a real estate purchase contract that was contingent on you securing financing to complete the purchase. If that is the case, then you would not be the one to write a letter. You can request that your lender to send a copy of the Adverse Action that was sent to you stating the reason(s) for your loan denial to the escrow agent. If you do not wish the reason(s) disclosed, the lender can write a letter stating that you were unable to qualify for said loan, without going into detail. Such a letter should enable the escrow agent to refund your earnest money, and cancel the contract.

How do I write a letter to void a mortgage contract based on finances?
You pay off the mortgage.