I am not a lawyer but, the buyer can file what is called a lis pendens against the property so that you cannot sell until the litigation they filed is done.
They can sue you for specific performance. If they followed what they had to do in the contract and you did not, then they should win monetary damages.
Think about the predicament you put them into. If the shoe were on the other foot......
Do get a real estate attorney if you do not want to proceed.