Contract for Deed problems

We bought a home contract for deed. In the contract it does state selling price,down payment & balance due with the agreement we will be assuming morgage at a certian banks name THRU the sellers as it states in our contract we are to make monthy payments directly to him for a ten year term. The whole contract is for a ten year term. Upon compleation of the terms as stated above in addition we agreed to pay all real estate taxes and equity due him(which is under 20,000 and provide homeowners insurance and a warrenty deed will be provided.Here is the problem. After five and a half years into our contract and reading our memo on our check each month I write 53 payments remaining only on contract for deed at:......He tells us there is a balloon payment due AFTER the ten years. NOWHERE in our contract does it even hint to such a balloon payment and the balloon payment he says is close in amount to 35,000.
If this was stated in our contract we would of never signed it.If there is a balance due on his morgage or a balloon payment due and we have completed the terms of our contract what could possibly happen? wedidsenddisputecert.mail HELP!

If your contract is as you state, then it is totally screwed up. First of all, a contract for deed does not entail assuming the Vendors mortage.
Secondly, the rest of your post contains various other problems that you need an attorney to review. Next time have a real estate agent or attorney review the sales contract.