If I give you $1 million (my own money) and want you to invest it for me, you were able to make from that investment 5% return every year, even though it's my money that I gave to you. Whereas I tried to invest that same money (my money) only to make a 1% profit return each year. All this means is you made better use of my money than I did.
The same example applies to a finished manga designed for children. Rinnegan being Madara's eyes is irrelevant really. He never shown any of Nagato's techniques, so he doesn't get any of them. It's really simple.