I send gift to my friend 10 $ but not transfer why

We are fellow users here on these forums, we won't know why.

The person that you sent it to has checked the spam folder on the email account that you sent it to as well as the inbox ? If it's not there either then you could try resending it (and double-check the email address) : http://support.apple.com/kb/HT1541

If you've also tried resending it and he still hasn't received it then try contacting iTunes Support and see if they can help with what happened to it : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption