What are the Shar'i violations in a commenda partnership contract where the buyer pays a sum of money to the company, and then the company refunds this amount after delivering the house, with one overt partner bearing responsibility and the remaining partners being covert?

There is no harm in one partner being responsible before others for his partners, but he should not be made to bear the loss alone, as loss is distributed according to capital. It is permissible to take a specific amount from the buyer as a guarantee and to return it upon delivery, provided that the contractor does not benefit from it, because that would be a loan that draws benefit, which is usury (riba).