Shaykh Ahmed An-Najmee mentions that it is on the correct opinion of it being permissible for a store keeper to sell building materials to a person who is building his house from a riba based loan he took. However, if it is a Muslim, he should advise him concerning the impermissibility of taking riba based loans.
As some scholars would say, there is a difference between فتوى (fatwa) and تقوى (taqwa), meaning between being cautious and between the actual verdict of permissibility.
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Mentioned by
AbdulFattaah Bin Uthman
Abu Fajr