I received a free item as part of a promotion – do I have to return it if I cancel my order or return part of it?
Yes, if you received a free item as part of a promotion that was linked to a specific minimum order value, you must return this item if your return means that the minimum order value is no longer met. Otherwise, we are entitled to charge you for the value of the free item or to offset it against your refund. The free item is a voluntary bonus subject to compliance with the promotion terms and conditions and is not a “gift” without consideration.
https://www.siku.de/en/Frequently-asked-questions/Onlineshop/I-received-a-free-item-as-part-of-a-promotion-do-I-have-to-return-it-if-I-cancel-my-order-or-return-part-of-it