Is the transaction of the "Tahweesh Society," which offers a known cashback amount in addition to the paid amount, considered usury (riba) and forbidden in Islam?

This is exactly usury (riba), which is a guaranteed increase in money due to deferment. Its meaning is the exchange of money for money on credit with a disparity, or immediate cash for deferred cash that is guaranteed to be more. Zakariya al-Ansari defined it as: "A contract involving a specific recompense, the equivalence of which is not known by the Shari'ah standard at the time of the contract, or with a delay in both counter-values, or one of them."