A hadith says: ?every debt that brings some profit it is interest?, so the profit gained from a debt is open interest whether in fixed period or outside the period, it is interest in any case. The bank people did not tell you the right ruling. According to Islam, it is unlawful for bank to give loan and receive extra money.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband