Must I donate the value of the prize I won in a drawing competition sponsored by a usurious bank, knowing that the prize's financiers are another company, but it is not certain if all the money came from them?

The permissibility of accepting a prize from a competition depends on the ruling regarding the drawing itself. If the drawing is of animate beings, then it is not permissible. However, if it is of inanimate objects, then there is no harm in it. Therefore, if the drawings entered into the competition are permissible, and the purpose of the competition is permissible, then there is no harm in benefiting from the prize, even if the bank contributed a part of it. This is because the bank's money is mixed, and some scholars permit accepting a gift from someone whose money is mixed, with certain conditions.