What is the expiation for one who inserts his finger into his wife's anus during intercourse?

The act mentioned is repugnant and goes against sound human nature. There is no specific expiation for it. Rather, it is expiated by sincere repentance with its conditions: abandoning the sin, regretting it, and resolving never to return to it. Among the things that help in regretting and not returning to it is understanding the ugliness of this sin and its consequences.