A contract of sale is basically a contract wherein you obligated yourself to sell a certain property at a specific time. More often than not, contracts to sell have a clause on what would happen in case the seller does not want to go through the process any more. If so, then comply with it to get out of the contract.
In case it doesn't have this provision, you have 2 probable outcomes on how this would proceed:
1) Ask nicely - Legally, the buyer already has the right to expect that you go through the transaction. To solve this as simply as possible, you could ask the buyer (nicely) if he would agree not go through with the transaction. If he says yes, then just make another document saying that both parties agree not to go through with the transaction, sign them, and you're off the hook. Don't forget to return the money you got.
2) Go through litigation - In case the buyer is intent on going through with the contract, he may file a case in court against you for specific performance. There, the court may compel you to go through with the transaction, or in case you really choose not to do so, the court will ask you to pay for whatever damages the buyer has incurred as a result of your refusal to comply.
I advise you do to everything to avoid this since this will entail more expenses on your part.
I hope this helps, and I hope you get to resolve your issue soon.