The easiest way to offer a discount for a specific payment method is to create a variant within the same product.
Here's how to do so:
Go to your products dashboard, then click a product title to open the "Edit product" slide-over (or create a new product if you don't have the product created already).
In the slide over, open the "Variants" section and create a new variant
For this variant, enable only the payment method for which you want to offer a discount, and set the discounted price.
Once done, save this variant.
You'll be back to the main edit slide-over
Here, select the first variant (that is now labeled "Default"), and edit its details.
For this variant, enable all other payment methods you'd like to accept, except for the payment method which you want to offer a discount. Set the regular (non-discounted) price.
Once done, save this variant as well.
That's all done! You've now got a product which has two different prices:
The original variant with the regular price and payment methods
The new variant with a discounted price for a specific payment method