If a consideration is agreed upon in line with a gift, its fair market value according to the general principles of the Valuation Act in the version of the Inheritance and Gift Tax Reform Act 2009 reduces the acquisition liable to tax. No differentiation is to be made between the real fair market value and the fair market value for Inheritance and Gift Tax purposes.
For more details see Koeniger, The German Inheritance and Gift Tax, 2017, p. 58 et seqq.