Your CPF savings for T-bills purchase under CPFIS would be deducted only upon the successful allotment of the T-bills. In so doing, CPF members will continue earning CPF interest on their CPF savings, in case full or part of their T-bills application is unsuccessful. The deductions are done after the auction date (i.e. the date where the allotment of your T-bills purchase has been finalised) but before the issue date of the T-bills.