I find this quite hard to explain but I'll try. In the first sentence, the object will be old at that exact moment in the future we're talking about. So if you imagine we're actually at the time when the person has saved up enough money. The model will be old in the present at that time. With the second example, imagine we're at the point when he has returned. The moment that they have forgotten about him will be in the past at that time.
I think that most native speakers don't really understand the rules and it just comes naturally over time. I hope this helps.