Is a commercial transaction where a sum is paid to a contractor to purchase goods and sell them at a predetermined price, along with receiving a check for the paid amount plus the agreed-upon profit, considered permissible (halal) or usury (riba)? And how can one extricate themselves from it if it is usury?

The transaction between you and the contractor, if the contractor guarantees the principal and a known profit, is invalid because it is a loan with interest. However, if the transaction is that you pay him the principal for him to buy and sell goods without a guarantee for the principal except in cases of transgression or negligence, and the profit is divided between you, and the loss is borne by the principal, then it is a lawful and permissible mudarabah. Likewise, if you authorize him to buy and sell goods for a fee or a percentage, and the goods are a trust in his possession, which he does not guarantee except in cases of transgression or negligence. There is no objection to taking a check for the original amount as proof, provided the money is not guaranteed by him.