What is the ruling on inserting a finger between the buttocks of the wife during intercourse (without entering the anus)?

The fundamental principle is the permissibility of a husband and wife enjoying each other, except for what the Sharī‘ah has forbidden, such as anal intercourse or intercourse during menstruation and post-natal bleeding. Allah, the Almighty, says: "They are a garment for you, and you are a garment for them." It is obligatory to avoid touching impurities without necessity. What was mentioned in the question does not fall under forbidden enjoyment, but one should be cautious lest it leads to something forbidden.