They should give you back the same amount as appeared on the refund tax form...
Also RMB 300 is not the correct amount..but now its too late i think as you are already back to HK ...
But next time, make sure to ask them again...
the point is are you still in france?
because once you leave the country, you cannot have the tax refund.
and they usually wont give you more $$ once you got it.