Frankly, it wasn't your fault. It's his for not doing sufficient research on his motherboard before proceed into buying your rams. You don't ask a shop to refund you if your motherboard cant support ddr400.

Anyway it's up to you whether you want to refund him or not. You are not obligated to do so. However if you are kind enough, you can ask him to post back the RAM to you and you bank in the RAM money excluding your petrol money (to be fair to you)