Q:

What solutions are there to do a refund of a transaction when sending monero xmr?

Since you can't see who sent you monero, what are some of the proposed ways of issuing a refund to users? Say I did a crowdfunding campaign and collected a bunch of monero. How would I refund the users that paid me if my campaign wasn't 100% funded?

A:

Monero's stealth addresses make it impossible for you to know the wallet address of anyone that sends you funds, unless they tell you.
Therefore you need them to prove that they sent you funds, and at the same time tell you the wallet address where they would like their refund sent.
There is a guide to proving payment here: https://getmonero.org/resources/user-guides/prove-payment.html

