You say your PO must be in multiples of 100. Is there a reason for using the fixed lot size? You should be able to get the results you describe by selecting the EX lot size and setting the rounding value to 100.
Standard SAP will apply the rounding value when you enter the PO quantity. I'm pretty confident of this because all the BAPI for PO creation includes a NO_ROUNDING parameter to supress this function.