Thank you for using Apple Support Communities. It's my understanding that you've got a $10.00 store credit but you're unable to purchase a ringtone. I'm happy to help.
First, confirm your credit balance using the steps in the link below:
View your store credit in the App Store or iTunes Store
If the credit balance is accurate, make sure the value of your purchase is less than the credit. If the value of the purchase is more, you will need to add another payment method. Please review the information in the following link:
How iTunes Store purchases are billed
If the issue persists, tell us what happens when you try to download a ringtone. Please include specific error messages and any troubleshooting steps you may have tried.
Have a great day