Whether you should or shouldnt have opened the envelope is now irrelevant.................... we can deal with that later................ either he is puutng money in or someone else is.........either way regardless of how you found out, there IS a problem.
Being noble, one could argue that you could take the document to him and say WE have a problem, your daughter is receiving money that is not hers, and how are WE going to deal with this.
or he is giving her money for reasons he does not want to explain to you.....
you have opened the mail, you do know about it FACT........... ooops sorry.............. but hey, now you need to understand what is going on.............
If there is nothing to hide, there is no reason for him to not explain...............
He explain, you understand............life can go on !!! Simples !!!!