German opposition parties have criticised Chancellor Angela Merkel's party after it received major donations from the family that controls BMW.
The funding boost for the Christian Democrats comes as Germany lobbies against tougher European Union caps on carbon emissions.
Ms Merkel's party said the three donations made on 9 October, and totalling €690,000, "have no connection to any political decisions".
The funds came days before EU environment ministers backed German demands to scrap a deal to cap EU car emissions that Berlin had argued would cost jobs and damage its premium car makers.
The donations came from Johanna Quandt, the widow of industrialist Herbert Quandt who is credited with transforming BMW, and her children Stefan Quandt and Susanne Klatten.
The family owns almost 47% of BMW.
Juergen Trittin, former co-leader of the Greens, wrote on Twitter that the family had "bought Merkel's climate policy on Oct. 9 for 690,000 euros".
Social Democrat Joachim Poss wrote that the short period between the donation and Ms Merkel's backing for the car industry were "grist to the mill for all those critical of party donations".
A spokesman for the Quandts said they had decided in January to donate to the CDU.
However, they said they waited until October to transfer the money because they did not want to get involved in campaigning for Germany's 22 September election.
The payments were registered with parliament, as required for all personal donations to parties of more than €50,000.
BMW produces heavier and relatively less fuel-efficient vehicles, meaning it would find it challenging to meet the proposed cap on carbon emissions of 95g/km for all new cars from 2020, analysts say.
The CDU said the Quandt family had supported it with donations for years, regardless of whether they were in government.
Ms Merkel's conservatives emerged as the dominant force in last month's election, but need a partner.
They are talking to both the SPD and the Greens, with the former seen as the most likely partner.
The Greens are strongly in favour of limiting CO2 emissions to 95g/km.