Hi  
Regarding the 50% cash must refund to cpf is referring to COV or the profit from selling hdb. 
Example: I get COV for selling my 4room flat : cash 35k   and total profit from selling the 4room flat is 90k , which mean hdb will bank in 90k cash for me. 
Just would like to clarify with you 50% is from cov 35k or profit from selling hdb 90k?