Hi Mr. Tan,
Are you referring to the situation where the Option is issued by seller, but the purchaser didn't exercise? Depending on whether it is HDB or Private Property, the Option Fee to be forfeited will vary (with private property higher), and the purchasers will just let the Option expire, and the sellers get to keep the forfeited option fee. Thereafter you (the seller) are allowed to sell to any other party, and neither party has any further obligation to each other.
Thanks is there an opportunity where I can work with you to assist further? Hope to speak soon.