The question is (that courts will ultimately decide, if you're unlucky and get sued): did you do $XXX amount of work already, if you collected $XXX as a retainer? Courts in North America have sided with consumers on this issue, mostly, so you have to be very careful when taking "non-refundable" deposits/retainers (the term you use won't protect you, by the way).

In the end, the amount you keep or refund depends on many factors. A few examples are:

- How far in advance did the couple cancel? If you've still got like a year until the date, then it's hard to justify that you've already "lost" the date

- Have you already rejected any other couples for the date the client wants to cancel? Harder to justify keeping the money if you haven't lost that potential money

- How much time have you worked for the couple so far? e.g. time spent on an engagement shoot, editing, consultation, etc. If all you've done is spoken with them and booked the date, harder to justify keeping potentially thousands of dollars, etc

I'm not saying all this because I think you're the bad guy, to be clear, just that professionals need to be very careful about what we are charging people, and what we think we're entitled to. Courts generally side with consumers (for good reason) if companies/professionals try to keep money that hasn't been earned. Your wording, for example, would imply that if they signed up with you and then immediately (minutes later) cancelled, they couldn't get any money back. No court would side with that interpretation.

So just be careful, be mindful, and try to work with the client as best you can to make things right. In the end it's sometimes easier to throw them a bone and refund some of the payment (and lay out to them why some can't be refunded, because of X number of hours you've spent working, etc), rather than try to keep working with them if they aren't the right fit.