The 10% per year is not an offset.
When you leave HK (and in principal not to return) you are allowed to take your MPF out but you only get 100% after working for 10 years at the company (this includes the years working for the same company in your previous country)
For each each year you get 10% so let’s say you leave after 3 years then you can only get 30%. After 10 years 100%.