Is there sin or participation in sin for the contractor who builds homes for citizens with usurious loan financing, knowing that these loans are the only way for citizens to build their homes?

It is not permissible for a Muslim to borrow with interest, because every loan that draws a benefit is usury (riba), and this falls under cooperating in sin. However, the borrowed money itself is not forbidden, so it is permissible to deal with it through buying, selling, and gifting. The prohibition relates to the stipulated increase on the principal of the loan, and the lender is the one who took the usury.