Your argument is flawed. Let me tell you why. In the case of
your example of putting money accidentally into my bank account,
there was no consideration from my part. If its an honest mistake,
I am legally obliged to return you the sum if Im not the intended
receipient of the sum.
However, in this case, HDB asked that she give up her stall in
exchange for the 18k. There is consideration involved and this is a
done deal. To ask for the 18k back after 4 years is akin to
reneging on your promise.
The appropriate example to use is that of a slimy businessman who
after paying money for the goods he received decides to to ask for
a refund 4 years later after he realises it was his mistake in
ordering the wrong goods.
Tell me, In what way did she beneift from the 18k when the 18k was
not at outright gift but a consideration awarded to her in exchange
for giving up her livelihood?
The facts are clear: HDB offered her 18k ,and in exchange, she duly
complied in the interest of HDB. HDB is clearly at fault and will
the lady does not have a legal obligation to comply with HDB's
request.