Positive Mitzvah 142
Collecting Loans given to non-Jews
Deuteronomy 15:3 "You may collect from the foreigner"
HaShem wants us to be careful not to be attracted to the lifestyle of non-Jews who worship idols. Our interaction with them should be formal and businesslike, without showing them special considerations.
Therefore, loans that have been given to a gentile should be collected despite the Shemitah year.