Is the husband who caused his wife's death in a car accident and paid her blood money to her family obligated, in Islamic law, to save a sum of money for his minor daughter as compensation for her, or is he permitted to use part of this money to buy a car, with the possibility of postponing the savings until she reaches maturity?

As long as you were at fault in the accident, you are required to offer expiation for accidental killing by fasting for two consecutive months, if you cannot find a believing slave to free. As for the blood money (diyyah), it is obligatory upon the 'Aqilah (male relatives on the father's side), and if they are unable or non-existent, then it is obligatory upon the Muslim treasury (Bayt al-Mal). If it is not possible to obtain it from the treasury, then the Hanbali school of thought holds that it is waived for the perpetrator, while the majority of scholars believe it is obligatory upon the perpetrator's own wealth. The blood money for accidental killing is paid in installments over three years. There is no harm for you in buying a car if you are in need of it, and you are not obligated to save money for the blood money until after you have spent on yourself in a reasonable manner. It is permissible for you to account for your daughter's expenses from her money inherited from her mother's blood money, because her maintenance is not obligatory upon you if she has sufficient wealth.