What is the Shar'i ruling on undeserved monetary excess, whether it be from an employer, or when purchasing a commodity, or when receiving change, or when phone credit is added by mistake?

You must return rights to their owners. It is not permissible for you to take someone else's money due to their oversight or mistake, whether it's something they gave you that wasn't rightfully yours, or something they took from you that was less than what they owed, or something deposited into your account by mistake. This is indicated by Quranic verses and Prophetic hadiths that prohibit consuming people's wealth unjustly, mandate the fulfillment of trusts, and warn against betrayal and taking others' money without right. However, if the owner of the money gives you an excess out of their own free will, there is no harm in taking it.