The Rebbe's guidance on debts, loans and interest:
Pay debts promptly and responsibly. Settle debts and return what was borrowed on time, especially when others are affected by the delay (IK 7844). Where debts are owed to an institution, a structured payment plan helps (IK 2830). Until debts are cleared, avoid non-essential expenses (IK 6461).
Interest: always use a heter iska. For any loan or business arrangement that may involve interest, arrange a heter iska to avoid any doubt, including when buying a home with a mortgage. All parties to an interest arrangement need to be careful and get proper guidance (IK 2256, 3129, 3246, 5482).
Halachic points: the distinction between a loan and a sale or gift, and when the prohibition of pressuring a debtor applies (IK 116); prozbul applies to verbal debts as well (IK 6518).
Gemachs: free loans. Supporting a free loan fund is a lasting merit, since it helps others while largely preserving one's own resources, and it is a great act of kindness that hastens the Geulah (IK 2305, 2417, 2480, 5099, 5132). The Rebbe himself arranged gemach loans for people in need (IK 1407).
Helping others: one who takes on debt in order to help others will not lose by it; Hashem repays (IK 881).