How do discounts created in GlobalTill flow into Shopify?
Integrations vary from store to store. Typically, discounts in GlobalTill flow into Shopify as below.
After a product is discounted (and the On Sale Now Price is populated), GlobalTill will push the GlobalTill On Sale Now Price into the Shopify Price and the GlobalTill Retail Price into the Shopify Compare at price. This will take place on the next sync.
In some cases, integrations are set up such that sale prices in GlobalTill do not sync over to Shopify at all. The use-case is to execute their own online sale from the Shopify side, independent of GlobalTill.
For more information, see the article How to create a sale.