The individual is under investigation or has been charged with a crime, and wants to offer information to law enforcement authorities in exchange for some benefit, such as dropped or reduced charges or an agreement for a lower sentence.
The proposal to give such information is referred to as a proffer. What's the translation of proffer in French? My guess would be "offre de preuve" but I wonder if there exists a more accurate legal term. Google Translate and DeepL didn't help.