Dealing with riba (usury, interest) is a major sin against which Allah has issued a stern warning. He says (interpretation of the meaning):

“O you who believe! Fear Allah and give up what remains (due to you) from Ribaa (from now onward) if you are (really) believers.

279. And if you do not do it, then take a notice of war from Allah and His Messenger but if you repent, you shall have your capital sums. Deal not unjustly (by asking more than your capital sums), and you shall not be dealt with unjustly (by receiving less than your capital sums)”
[al-Baqarah 2:278]

“Those who eat Ribaa will not stand (on the Day of Resurrection) except like the standing of a person beaten by Shaytaan (Satan) leading him to insanity. That is because they say: “Trading is only like Riba,” whereas Allah has permitted trading and forbidden Riba. So whosoever receives an admonition from his Lord and stops eating Riba, shall not be punished for the past; his case is for Allah (to judge); but whoever returns (to Riba), such are the dwellers of the Fire — they will abide therein”
[al-Baqarah 2:275]