Coming in late to this...
Corolla, you mentioned that you think you should be entitled to some of your friend's gain because, without your money, he would have had to make different choices about his property.
However, you just gave this guy a loan of money, you did not co-invest in his venture - if he had lost money on his land transaction, would you reduce the amount you wanted him to repay? It sounds to me that you actually want more than "interest" - you want a cut of his pie.
It's like loaning someone a tenner to bet on a horse that comes in at 100/1. The person you loaned the money to may give you extra back as a thank you, but it would definitely be churlish to ask for it as of right.
Just be happy that your friend has made some money out of his property and be glad that you were in a position to help him do so.
Sprite