Your stamp was attached to a Zahlkarte. It reads, "Zum Aufkleben der Freimarke durch den Absender (Gebührensätze umseitig)" which roughly translates as "Sender is responsible for attaching postage (rates on the reverse)".
I'm not 100% clear on the concept, but as I understand it, a Zahlkarte was a form of payment card processed through the post office. The money sender would pay the post office, which would deposit the money in an account for the recipient. The Zahlkarte was then sent informing the recipient of the transfer. The reverse of the Zahlkarte listed the postage rate, which was dependent on the amount of money being transferred.
Here's a random image I found on Google. Note the postage block at top-center that matches the reverse of your stamp.

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