In order to purchase items by a unit of measure that differs from the selling unit of measure take the following steps to create two Managed Products for the item.
Create a Managed Product using the Special Product type for the bulk UOM item, for example a box of 100 of your item.
Create a managed product for the Each quantity of the item using the Hardware or other appropriate type if the item is supported in our price books.
Order the bulk UOM, ex. Box 100, item from the vendor and receive it. Once received process a Product Count that includes these two items, the Box 100 and and Each. Decrease the counted quantity of the Box 100 to zero, or whatever unit of 100 you plan to transfer to the Each item. Set the counted quantity of the Each item at a value increased by 100 or whatever unit of 100 you are transferring. Save the Completed Product Count and you will now have Each items available to sell.