Is it permissible to benefit from a large financial prize from a usurious bank, considering it a loan without interest, to be repaid in monthly installments, given the absence of Islamic banks or loans in the country?

You did not clarify the reason for the prize, but if it is a pure gift, then there is no sin in accepting it and benefiting from it, and there is no need to return its equivalent. If its reason was depositing money with him, then it is usurious interest that must be disposed of by spending it on the welfare of Muslims, the poor, and the needy. One should not consume from it unless they are poor and in need, and it should not be returned to the usurious bank. It should be noted that dealing with usurious banks is not permissible except out of necessity and the absence of an Islamic alternative.