Transfer purchase without syncing to computer?

I have an iPod touch with an app that I bought a long time ago on it. The iPod is currently synced with another computer and I want the app to sync to my iPhone on my computer without syncing to my computer, as it will remove everything from it. The problem is, though, that iTunes doesn't think I've bought the app already and it charges me for it, but I have no money to buy it again. How can I retrieve the app from the iPod without the other computer and put it on my iPhone?