@Walter

First of all that's very honest and kind of you!

If you want to send them the difference, you can do this by clicking the pixcoin symbol () under your avatar. This will take you to your pixcoin page. Then find the Pixcoin Sending/Trading section (third section, under inventory expansion) and you'll see a search box. Type in the username you want to send pixcoins to, then click Search User. The user should come up and then you can enter how much you want to send. Then hit Send Pixcoins and you're all set.

Let me know if anything I said was confusing or if you have further questions! I'll be happy to answer! :)