Ushers making change from the offering plate

I was recently told that, from the loose tithes/offerings in the offering plate, our ushers have been making change for people who do not have the exact change amount for their tithe.

For instance, if a parishioner only had a $100 bill, but that person was only going to give a $20 tithe, the ushers would give $80 change from the loose cash in the offering plate.

There are red flags and warning sirens going off in my head over this, but I need to narrow down the specific reason(s) why this is not something they should do, other than just saying it is not best practice.

(We do have tithe collection procedures, but they begin with having 2 ushers count the tithe and do not include anything that would address this issue.)

Or am I over-reacting and there may be a control for making this an official procedure if necessary?