All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Riba (interest and/or usury) is a grave and major sin but it is absolutely lesser than Shirk [associating partners to Allaah] because there is no greater sin than Shirk. However, a person who deals with Riba is sinful and deserves the Punishment of Allaah which is being prevented from His Mercy and entering Hellfire if Allaah wishes to punish him. Nonetheless, he is not an apostate and he is not considered as committing Shirk unless he believes that Riba is lawful after it has been confirmed to him that it [Riba] is forbidden.
Allaah Knows best.