The beginning of Soviet and German cooperation goes back much earlier, to early 1970s. The famous policy of "Wandel durch Handel"

"Wandel durch Handel (WdH, German for "Change through trade"), also known as Wandel durch Annäherung, is a political and economic notion, mostly associated with German foreign policy, of increasing trade with authoritarian regimes in an effort to induce political change. Although most strongly associated with Germany, similar policies have been pursued by several Western countries."

https://en.wikipedia.org/wiki/Wandel_durch_Handel

For example, West Germany has helped with building Soviet gas network.

https://ost-ausschuss.de/sites/default/files/pm_pdf/German-R...

All subsequent goverments of Kohl, Schröder, Merkel supported expansion of gas imports from Russia. It was cheap and reliable even during Cold war.

And little bit of money from Gazprom to politicians always helped.

https://correctiv.org/en/latest-stories/2022/10/07/gazprom-l...