Where I live, one can go to any bank and buy rolls (or cases!) of pennies, and one can also redeem such rolls. Merchants do both of these often.
I have noticed that rolls of pennies occasionally contain foreign currency of similar size and appearance. I'm in the US and we see Canadian pennies sometimes; presumably they're mixed into the general money supply and often people don't notice them. I don't know what procedures banks use to check/validate the rolls of coins they distribute, but I assume it doesn't involve anybody inspecting individual coins and is probably based on weight and length.
My question is: if you have a large number of pennies that you want to deposit, does halacha require that you inspect them all first and weed out any foreign ones, or does the fact that the banks don't seem to do this either mean that that's just a risk we all take? Does the answer depend on likelihood, for example one who just took a trip to Canada and has a pocketfull of change has to check but in general people don't?
This question was prompted by a project in my synagogue's school to collect 304,805 pennies as part of writing a new sefer torah. I'm asking out of curiosity, not as a decision-maker for those deposits.