All Discounts will be earned, calculated and vested as set forth in Exhibit A . For purposes of calculating the Discounts hereunder, Qualified Gross Purchases of Amgen ESAs by any Dialysis Center Purchaser shall be deemed to be made on the date of invoice by an Authorized Wholesaler or Amgen pursuant to Section 3.6 to any such Dialysis Center Purchaser. The Discounts (other than the Base Invoice Discounts) shall be paid in arrears by electronic funds transfer using information provided to Amgen by Dialysis Center as necessary to enable payment. Amgen Inc. hereby guarantees Amgen’s obligations to pay all Discounts earned by Dialysis Center hereunder.