Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos) By the Chofetz Chaim

This book lists the Torah mitzvos that can be observed today

Second Section – the Negative Commandments

57. It is a negative commandment “do not demand payment of a loan that has passed through the shmittah year”

As it states in Devarim (Deuteronomy) 15:2, “one should not pressure his friend or his brother because the shmittah year has been proclaimed.” All of this applied during the time when we had to custom to have a Jubilee year, but nowadays, since releasing of loans from the shmittah year is from the Sages, a person does not break a Torah law if he transgresses this negative commandment, but only a Rabbinic law.