If you want to consider dispensing an undesired generic a dispensing error and risk incurring any civil and/or criminal penalties that arise from dispensing errors, go for it.
http://www.deadiversion.usdoj.gov/faq/prescriptions.htm
Question: Can an individual return his/her controlled substance prescription medication to a pharmacy?
Answer: No. An individual patient may not return his/her unused controlled substance prescription medication to the pharmacy. Federal laws and regulations make no provisions for an individual to return the controlled substance prescription medication to a pharmacy for further dispensing or for disposal. There are no provisions in the Controlled Substances Act or Code of Federal Regulations (CFR) for a DEA registrant (i.e., retail pharmacy) to acquire controlled substances from a non-registrant (i.e., individual patient).
The CFR does have a provision for an individual to return his/her unused controlled substance medication to the pharmacy in the event of the controlled substance being recalled or a dispensing error has occurred.
http://www.deadiversion.usdoj.gov/fed_regs/rules/2010/fr0331.htm
The Controlled Substances Act is unique among criminal laws in that it stipulates acts pertaining to controlled substances that are permissible. That is, if the CSA does not explicitly permit an action pertaining to a controlled substance, then by its lack of explicit permissibility the act is prohibited.