Thanks guys for your kind advises. I spoke to my accounting firm to seek for advise before asking some legal advise. Apparently, my accountant has some knowledge on this issue. And well, I think I 'll have it as "another lesson to learn"

as there is no concrete evidence to proof it that the client has legal obligation to pay me for the amount due, mainly I do not have a "black and white" agreement being signed!
Fortunately, it was not a big sum, just a couple of hundreds. I am learning from this episode, and start to draft out some basic contract for future use.