Currently the option of adding a discount into a Purchase Transactions in an enhancement, namely
Discount Item applied at transaction level: (Issue 180038) for purchase transactions
Non-posting Discount Items: Issue 154080) for purchase transactions
As a work around you could give the below a try:
1) Create a Discount Inventory Item ( You can do this from Lists- > Accounting - > Items - > New -> Discount . Name it as Discount for Purchse and put the percentage needed, and make sure you map it to the right account. Account should be an Expense Account
2) When you create the Purchase order, you can insert this discount line.
or if you have multiple items, you can put a Subtotal line and then put the discount line
3) When you take the print, you can see your items and the bulk discount applied
*The only issue with this is on the PO print, the discount item is shown as a transaction line and not in the Discount Item portion of the Summary Total for the transaction.. The reason being all items entered on the line item including discount will be included on the computation of the Subtotal in the Summary total. Only when the discount is entered on the separate Discount fields below the Items tab ( which is not available for Purchase Orders) will be displayed on the Discount Item portion of the Summary Total of the transaction.
Non-posting Discount Items: Issue 154080) for purchase transactions
As a work around you could give the below a try:
1) Create a Discount Inventory Item ( You can do this from Lists- > Accounting - > Items - > New -> Discount . Name it as Discount for Purchse and put the percentage needed, and make sure you map it to the right account. Account should be an Expense Account

2) When you create the Purchase order, you can insert this discount line.

or if you have multiple items, you can put a Subtotal line and then put the discount line

3) When you take the print, you can see your items and the bulk discount applied

*The only issue with this is on the PO print, the discount item is shown as a transaction line and not in the Discount Item portion of the Summary Total for the transaction.. The reason being all items entered on the line item including discount will be included on the computation of the Subtotal in the Summary total. Only when the discount is entered on the separate Discount fields below the Items tab ( which is not available for Purchase Orders) will be displayed on the Discount Item portion of the Summary Total of the transaction.