Discounts

Sir can you please help me how to treat Sale tax on purchases. Like we have IFRS-15 for Revenues. Scenario 1: Supplier thought we will not achieve cash discount according to past experience so he will send us invoice of $200 (no deduction of cash discount of $40). Whereas, we achieve the cash discount by paying in time so supplier will decrease sales by $40. On other hand, customer will record it as discount received of $40. Am I correct? Scenario 2: Supplier thought we will achieve cash discount according to past experience so he will send us invoice of $160 (after deducting cash discount of $40). However, we had not achieve so supplier will increase sales by 40, on the other hand what will we do as a customer. Will we increase our purchases by 40? Please help me with this. Thank You.