Is it permissible to convey reward to one who slaughtered for other than Allah?

Q: This questioner says: His father slaughtered for other than Allah, as was reported to him about this, and he now wants to give charity on his behalf and perform Hajj on his behalf. He attributes the reason his father fell into this to the absence of scholars, guides, and advisors for him. What is the ruling on all of this?

A: If his father is known for goodness, Islam, and righteousness, it is not permissible for him to believe those who report otherwise about him, provided those people are not known for their integrity. It is recommended for him to make supplication and give charity on his behalf until he is certain that his father died upon shirk (associating partners with Allah). This certainty is established by the testimony of two or more trustworthy and upright witnesses who saw him slaughtering for other than Allah, such as at the graves of the dead or others, or who heard him calling upon other than Allah. At that point, he should refrain from making supplication for him, and his case is left to Allah . This is because the Prophet (peace and blessings be upon him) sought permission from his Lord to seek forgiveness for his mother, but Allah did not grant him permission, even though she died in the pre-Islamic age of ignorance upon the religion of the disbelievers. Then he sought permission from his Lord to visit her grave, and Allah granted him permission. This indicates that one who dies upon shirk, even if ignorant, is not to be supplicated for, nor is forgiveness sought for him, nor is charity given on his behalf, nor is Hajj performed on his behalf. As for one who dies in a place to which the call of Allah has not reached, his case is left to Allah (Glorified and Exalted is He).
The correct opinion among the scholars is that he will be tested on the Day of Resurrection; if he obeys, he will enter Paradise, and if he disobeys, he will enter the Fire; due to authentic hadiths that have been reported regarding this.