What avast version are you using 4.8 or 5.0 ?
Send the sample to virus (at) avast (dot) com zipped and password protected with the password in email body, a link to this topic might help and false positive/undetected malware in the subject.
Or
If 5.0 - Open the chest and right click on the file and select 'Submit to virus lab...' complete the form and submit, the file will be uploaded during the next update.
If 4.8 - Add the file to the User Files (File, Add) section of the avast chest where it can do no harm and send it from there. A copy of the file/s will remain in the original location, so you will need to take further action and can remove/rename that.
Send it from the User Files section of the chest (select the file, right click, email to Alwil Software). It will be uploaded (not actually emailed) to avast when the next avast auto (or manual) update is done.