The closest thing I found to what I need is here, the problem is, even though it's only 8 months old, it appears outdated. I see there is an "outstanding balance" in the PayController, but I have no idea how to allow a customer to pay a specific amount at the gateway.
I created an adjuster that discounts a specific amount, so I am able to bill the correct amount at the time of payment, but the amount that I discounted needs to be reapplied to the order in order to be billed at a later date, which I haven't found a way to do this as every possibility I've seen/tried is either a read-only or it's another adjuster which I can't get to fire at the correct time. I tried to put that amount back on with an adjuster with the EVENT_BEFORE_ORDER_COMPLETE but then craft obviously sees the order total has changed and kicks me back to the payment form with my new adjustments, which negates my old one.
I just need to know how to set the amount a customer must pay at the gateway. Is there a specific way an adjuster needs to be set up in order to do this?