A claim to a compulsory portion (§§ 2303 et seqq. Civil Code) which was not asserted by the deceased is part of his estate and is considered an acquisition by way of death by the heir according to § 3 para. 1 no. 1 Inheritance and Gift Tax Act (IGTA). The assertion of the claim to a compulsory portion by the heir is of no relevance.
Due to the universal succession of § 1922 Civil Code, the heir fully steps into the legal position of the deceased which inter alia includes the right to assert the claim to a compulsory portion. Economic aspects do not apply since § 3 IGTA explicitly refers to the succession regulations of the Civil Code.
For more details regarding the claim to a compulsory portion see Koeniger, The German Inheritance and Gift Tax, 2017, p. 30, 34 et seqq.