Doubt you'll get credit for a Tinfoil Barb, they have no value at all to the shop. Take up space, arent worth much and get huge. Rhere is one MA currently trying to shift 40 x 8-12" Tinfoils because people take on fish they cant look after or get bored of them and return them and so the shop ends up with loads of them.
If it was something the shop could actually make money from like big clown loaches or something, sure, you ought to get some credit... but when you have a species that most shops will give away or ask for a £10 rehoming fee... whats the point in giving credit for a fish they dont want? They would be doing YOU a favour in taking it off you... not the other way around.
People treat shops like swap shops and rehoming centers.... its no wonder more and more shops are taking on a 'no returns' policy (with exceptions to customers that bought fish within the week or two limit).