The student union fee (currently €12 per semester) is part of the semester fee which you pay to the University.

If you terminate your enrollment you may be entitled for a refund.

If you terminate your enrollment within a month after the start of the lecture period of the semester you paid the fee for, the student union fee will be refunded. (§ 4 Abs. 2 BeitragsO VS)

If your termination date is before the start of the new semester (earlier than Oct. 1st / April 1st) you will automatically get a full refund from the University.

If your termination date is Oct./April 1st or later, but no later than one month after the start of the lecture period (Oct./April 14.) you need to apply for the refund. 

To receive a refund, please submit an informal application, including your full name, address, bank details, and a scan or photo of your „Termination of enrollment“-confirmation, via email to buero(at)vs-tuebingen.de. 

Alternatively, you can submit your application in writing, along with the above information and supporting documents, to the office in the Clubhaus (Wilhelmstr. 30, ground floor). 

Sample mail

Subject: Refund of the student union fee – Termination of enrollment

Dear ladies and gentlemen,

I hereby request the refund of my student union fee (according to § 4 Abs. 2 BeitragsO VS).

Name: [first- und surname]
Adresse: [address]
IBAN: [IBAN]
My enrollment was terminated by [datum of termination].
Attached you find the scan/picture of my confirmation.
Please refund the fee to the account above.

Best regards,
[Name]

The refunds will be processed as soon as possible, but not before the respective deadline (usually end of Nov./May).

This applies ONLY to the Student union fee, there are different regulations for the tution fee / administration fee / Studierendenwerk fee.
Further information on termination of enrollment provided by the university: 
https://uni-tuebingen.de/en/study/organizing-your-studies/semester-and-study-planning/termination-of-enrollment/