Free gifts are always tied to the purchase of a specific product. If you keep the associated product, you are accordingly allowed to keep the gift. However, if you return the item, the gift must also be returned.
Exceptions to this rule:
Excluded from this rule are naturally the free samples and the gifts marked as "Try Me" on the invoice. You can keep these in any case.
All information on how to return an item can be found in our article:
How can I return an item?