The Chase T&C says
Save 20% on Points Purchases: Save at least 20% on points purchases when you pay for those points with your IHG One Rewards Premier, Premier Business or Traveler Credit Cards. Point purchase amounts are subject to change by IHG One Rewards. These points are Non-Elite Qualifying points. Upon completion of the points purchase, you agree that the total dollar price of the points, minus the discount, will be immediately charged to your credit card. The cost for the points purchased is non-refundable. The 20% discount cannot be combined with any other points purchase offer. Point purchase transactions managed by Points.com
If you buy without an upfront discount, I would not expect Chase to give you a 20% discount later