How can I ensure that the confidential data is preserved?

The ideal promise that has the highest credibility is a commitment from the provider of the confidential data to maintain the data for a number of years.

A second-best solution requires that you inspect your data use agreement. Can you archive the data in a robust fashion (using whatever tools your university has)? Some DUA allow for that, others don’t. The promise then becomes that you/your university will guarantee persistence of the data for X years.

A better variant of that is when the DUA allows you to share the data with others that have also demonstratably signed a DUA with the provider. The data provider controls the access, you (your archive) provides the data.