If you need a Svara to be Mechalek between Davenning a Nedava and other Halachos of Shabbos, you can say something similar to what Chacham said and discarded. The idea of not Davenning a Tefillas Nedava is because there is no Nedava on Shabbos. It is not directly connected to Shabbos. It’s just not a Zman for that Teffila.
If so, as long as it is day and you could have been bringing your Nedava now, your Kabolas Shabbos shouldn’t affect the Zman. The only problem with this is the Limud Zchus for those that don’t repeat Krias Shma after Tzeis.