Is it permissible to pray behind an imam who issued a fatwa allowing the sacrifice of an animal to a saint for the healing of a possessed girl, and then ate from this sacrifice?

Slaughtering for the occupant of a grave as an act of drawing closer to him and venerating him is polytheism (shirk). It is not permissible to eat from the animal slaughtered by a polytheist, or that over which the name of Allah was not mentioned. Slaughtering for the worshipped entity is the utmost act of humility and submission to Him. It is not permissible to slaughter for other than Allah, nor to mention other than Allah's name over slaughtered animals. If the Imam believes in the permissibility of slaughtering for saints as a form of veneration and drawing closer to them, then this is a polytheistic innovation (bid'ah kufriyyah), and it is not valid to pray behind him. However, if the slaughtering at the grave is for Allah Almighty with the intention of feeding the poor, and not out of veneration for the saint, then the slaughtering is an innovation (bid'ah), but it is permissible to eat from it and to pray behind someone who commits such an innovation.