How do I set an issue unit of measure different than the item's primary unit of measure?

Summary:

We have some gowns in inventory that we order by a case of 4 boxes. Each box contains 10 gowns but we do not issue out by the each. For the sake of tracking cost per item we would like to have the primary unit of measure set to EA but have the option to issue out of inventory by the BX.

We are using Interclass unit of measure and have all three units of measure set up, however when I go into the purchasing tab of the item master record the "Unit of Issue" field does not allow me to select anything other than EA as the option.