If you want to understand a little more about the .RET files at a glance here .
Basically, the 'Non-generated' bank return files , they are returns!
The banks, in your case, the Santander are responsible for providing this information and should be extracted from your website. (the first link demonstrates how to extract the file and the second one a bit of use)
What your application should do is: Read the bank's return files.
With this it is possible to know if the affines were paid or if they are invalid (Followed by the codes that indicate the reasons for this, so that they can be corrected properly)
Access the Your bank's website
Click "File Transfer" and then in the Return option, click "Consult".
If the following screen appears, select "Simple Transfer" and then "Customize"
Enter start date, end date and then click the "Submit" button.
Simply Save to your computer.
Footstephasbeenremovedfromthefollowing CNAB-and-import-to-Vindi>> page and contains 7 steps, but the last 2 can be ignored because they are specific to a system.