With the awkward language differences, I don't understand your message 100%. (By the way, I love Singapore, especially the great food in the hawker markets! I have friends who live there.)
If you had the Flexible cancellation policy in effect when the guest booked, that wouldn't make any difference with respect to my previous reply. The guest's only charge if *they* were the ones who cancelled should have been AirBnB's 6-12% fee. LIkewise, if you were the one who cancelled, the guest should not have been charged anything, and *neither* should you.
The issue of transferring payment to another booking is irrelevant.
The key thing here is that the guest *might* be trying to scam you. Do NOT offer to pay the guest anything. Tell them to work with AirBnB if they have any issues about their charges. Their issue is with AirBnB, not with you.
If you find this helpful, consider clicking on the Thumbs Up button near the lower right. Thanks!