I have to agree with Paul here. It does seem like you're asking how to get a cookie from the jar when there are no cookies in the jar. Impossible.
What you need to do in this situation is prevent a cascading failure. That is, if that payment system goes wrong or goes missing then your application is not taken down with it. You need to be able to handle that scenario and make your application respond accordingly. First and foremost you need to configure wait timeouts whenever you connect to an external system so that you don't get stuck waiting forever on a dead resource. Another technique you can apply here is to use a mechanism called a "Circuit Breaker". This will stop your application trying to access a dead resource after x number of failed attempts and then wait for some timeout before trying again. This prevents your application spending time and resources trying to connect to a resource that it already knows is probably dead. "Fail fast" is the buzzword for this one.
In summary, you cannot guarantee 100% uptime and correct behaviour of any external system. Your application needs to deal with this and respond accordingly.
I would highly recommend getting hold of a copy of Michael T Nygard's "
Release It!" book which is a goldmine of techniques on dealing with software integration realities.