Rein here from Shopify. I see you are looking for a way to offer discounts based on the payment gateway selected at checkout.
There would actually be no way to directly manage that within Shopify. What I would suggest is perhaps utilizing some sort of promotional rebate system whereby you refund X% depending on the gateway chosen manually.
Rein | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog