Welcome! Assuming the male has been neutered for over a month (otherwise, he could still have viable sperm), then I have heard of this working but you may find some issues that you may need to be prepared for. Like a “false” pregnancy – she would not get pregnant, of course, but if he mounts her, her body may fool her into thinking she is and she can go through a whole three week period of nesting which can be rather stressful which can be accompanied by aggression. Also, you may find some more territorial marking etc. (but this usually happens anyway regardless)
Sometimes scratching at a cage can also be a form of aggression but you are there, and will be able to hopefully see the real intent.
So, I think your idea of starting in a neutral territory is best. So just take it slow and go through the bonding steps. The worst situation is that you are just back where you started but if it works, then Rupert may be happy again with a new best friend. It’s worth a try. However, if you do see repeat false pregnancies then you would need to be prepared to get her fixed asap to prevent this.
FYI: Also, since you are saving up, I would recommend doing the unfixed female first since they have a high rate of cancer of their reproductive organs.
Good Luck and Keep us updated!