Is it permissible for someone on whom a kafarah for accidental killing is incumbent for six people, and he is unable to fast for six consecutive months, and he does not possess the cost of freeing slaves, which amounts to 42,000 Saudi Riyals, to move to the option of paying the blood money to the best of his ability?

If you caused the death of six people, you are required to pay expiation for each person, which is the freeing of a slave. If you cannot find a slave to free, then you must fast for two consecutive months for each person killed. It is not permissible to give money in lieu of freeing a slave. If you are unable to fast, scholars differ on whether feeding sixty poor people for each expiation is sufficient, or if this obligation is waived due to your inability. Expiation is not waived if the victims' guardians pardon the blood money, because expiation is a right of Allah.