It's a good question and it's a question where people go along with what they assume is the answer rather than actually think it through. The assumption made is that if you offer something that was in your possession (i.e. a coin in the till of an employer) and it is refused, it remains in the possession of who had it previously (the employer). There's some logic in this.
However, legally you could argue that the change is owned by the consumer and when they say, "I do not want it" it is either up to them to decide what to do with it (i.e. give it to either you or the store) or leave that decision in your hands because you currently have their change in your hand. If you were out on the street and they said they didn't want 50c and walked off leaving it with you, most people would argue you could keep it. It's little different than the situation in the store. The transaction to buy a product has already taken place. That their 50c is the result of a transaction with your employer is of little consequence. If they decided only when they walked out of the store that they didn't want the 50c and gave it to a tramp, would your employer charge the tramp with theft?
For the sake of fairness, though, I would say an equitable way of handling things would be that if the change never left the till, you should treat it as your employers. If it got to your hand and the customer said they don't want it and you then say, "Can I take it as a tip?" and they say yes, it is definitely yours. Even if you don't ask about it being a tip, I can say you have a case.
As a kneejerk reaction, people will say you did wrong. But, putting some thought into it, the timing and location of when and where a customer decides they don't want some of their own money doesn't make it belong to your employer.
