Under German law an acceptance that departs in content from the offer is not an acceptance at all. It counts as a rejection combined with a new offer — § 150 BGB. The same applies to an acceptance that arrives too late, and a rejection extinguishes the original offer outright under § 146 BGB.
Ja, aber is legally a no. The original offer is gone, and the roles reverse — the person who made the first offer is now the one deciding.