Even if CISG is applicable (Art. 1 I, Art. 2 CISG) the parties can exclude its applicability either entirely or in parts or they can modify its regulations:

"This treaty shall be governed by German law excluding CISG"

Have to do it expressly, because the CISG is a part of a national law!

Exclution on the whole of CISG and varying the effect of its substantial rules: Art. 6 1 CISG