Is it permissible to repay the debt of the deceased woman to the relatives of the deceased creditor, or to his heirs, or to his acquaintances, if the questioner is unable to reach the heirs of the creditor?

The debt owed by your mother does not lapse with her death. It must be deducted from her estate before its distribution. If she has no estate, then it is recommended for you to pay it on her behalf. If the owner of the debt dies, it is obligatory to pay the money to his heirs for them to divide it according to Islamic law. If it is impossible to find them and you despair of locating them, then donate the money on behalf of its owner, and your obligation will be discharged. If you find the heirs later, inform them of what you have done. If they approve the charity, then so be it. If they demand the money, then pay them an equivalent amount. The reward for the charity will be for your mother if the debt is paid from her estate, and for you if it is paid from your own funds. The money is to be returned to the heirs of the creditor, and if they are not found, it is to be given as charity.