Lilian Tintori on Saturday posted a photo on Twitter of herself at Caracas' international airport holding a document from migration authorities ordering the seizure of her passport before she was to board an afternoon flight.
No explanation was given, but the move came a day after she was ordered to appear before a judge on Tuesday to answer questions about the 200 million bolivars found in her car.
The amount equals about $60,000 at the nation's weakest official exchange rate, or $10,000 at the widely used black market rate.
She denounced the probe as politically motivated, pointing out in a video it was not a crime to have cash in one's possession.
Tintori said the money was to pay for family emergencies, including the hospitalisation of her 100-year-old grandmother.
"The evidence is clear why the dictatorship is stirring the pot against me," Tintori tweeted.
"They want to keep me from talking about the humanitarian crisis we are living in Venezuela."
While it is not clear what possible crime Tintori is being investigated for, some government supporters have accused her of using the funds to finance "terrorism" - a term they frequently use to describe violent protests that have rocked Venezuela - although they have presented no evidence.
Tintori is the wife of Leopoldo Lopez, who served three years of a 14-year sentence for leading violent anti-government demonstrations in 2014, before being released from a military prison and placed under house arrest in July amid destabilising protests against President Nicolas Maduro.
Lopez's trial and conviction were marred by irregularities and have been condemned by numerous foreign governments and the United Nations.