Is it permissible for a Muslim to deal in usurious transactions in a society where the economy is based on Riba (usury/interest)?
A Muslim may not deal in usurious transactions at all even if the society's economy is based on Riba. This is based on the general nature of Nusus (religious texts) prohibiting Riba. Moreover, such a person has to resist such an evil as much as possible. If he cannot do so, he has to leave this community so as to abandon such an evil place and for fear that he might be affected by them.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.