How do I calculate a loan for a sale on contract?

We have a house that we bought to rehab and sell. The idea was to make a profit, but it is not working out that way, real estate values have gone down, etc. Now, we just want to sell the house and get out from under the payments we have. We are trying to sell on contract, and I need to find a way to calculate the payment, using simple interest, on a 12 mo contract with the balance to be paid off after 1 yr. We also want to allow the buyer to accumulate a down pmt to help get a mortgage. Can anyone help me with this.How do I calculate a loan for a sale on contract?
It sounds like you have a buyer who can't get a loan right now. Most sellers would offer the house on a lease option where the ';buyer'; gives you a higher than normal deposit which you hold as security that he/she will buy in 12 months when he can get a loan and you lease the property to him during the 12 months. If the ';buyer'; secures a loan after 12 months the deposit given to you would be applied to his down payment, but if he failed to secure the loan he would lose it to you completely.