CY731782023DE is still waiting for recipient to make customs clearance. I guess you are shipper, right? So your words "I paid the correct customs duties on the package and the goods exemption limit was not exceeded" is a little bit strange.
1)Shipper can not pay import duty in China. So how did you pay the China import duty? If you paid duty to receiver, you must be scammed.
2)Goods duty exemption is subject to recipient having made customs declaration. Obviously your recipient didn't make customs clearance. So China Post will estimate the import duty by assessment.

Is this a return for refund?

1)Your customs declaration in Germany is useless in China import procedure. Recipient must make customs declaration in China.
2) " The value is less than 150 euros. I assume that the dealer in China will not have to pay any additional fees" is TOTALLY WRONG! China duty exemption limit is RMB800 (about EUR96). Most important is that recipient must made customs declaration. If he/she has the proof that goods was shipped from China before, NO need pay import duty !

Some bad sellers don't want to give refund to buyer. So they often play the game and not make import customs clearance in order NOT to get return item delivered.

If this is your case, you need open dispute to your marketplace (Amazon, Aliexpress, ebay, TEMU etc). If other buyers filed similar dispute against the same seller, he/she will have trouble.