You may or may not have noticed. MBF stands for Money Backup File.
in your folder with your Money files, these are probably files that came to exist when you restored a *.mbf previously. Note they will be bigger than your other *.mbf files if your normal backups are compressed.
attrib *Backup*.mbf
will show the attributes. Expect R, which means Read-only, which indicates you cannot delete them.
attrib -R *Backup*.mbf
will clear the Read-only attribute. Like most things in Windows, this is not case-sensitive.
del *Backup*.mbf
can now delete those files, if you like, to reclaim some space. Careful...
I put my scheduled backups onto a USB flash drive. That is mainly to not have my main drive failure lose all of my backups.