When a shadchan makes a match for a couple, the halacha is that he must be paid. This is generically called sirsur, which means “broker,” and the Shulchan Aruch stipulates that a sirsur must be paid. After giving a list of examples, the Rama (CM 264:4) says that any deed or favor a friend does for you, you must pay him and you cannot say that you undertook it for free since we never made up beforehand.
What if two people are in a fight and after much effort with both people you manage to bring them together again and settle their differences? Must they pay you if you did not stipulate initially that you would like to be paid?
The Shevet HaKehosi (3:310) says that although the Nesivos HaMishpat says that this is only for something positive, but saving him from damage does not earn you any fee, nevertheless, in this case you are doing something very positive: you are making him and old-new friend.
Even so, says the Shevet HaKehosi, in this case you need not pay him. Since it is not the minhag to pay for this service nowadays, you can’t demand money even if one or both of the parties approached you to get involved. The mitzva of making peace among friends is so great that unless the broker specifically said up front that he would like to be paid, he is not paid.