I agonized over this all day. 
Thank you ALL for your perspectives on this.
He and I chatted about this when I picked him up from school and we decided together that it would be fair if we did this: he'll surely get his two $5 from the last two weeks allowance (not a $10, which is my decision to avoid confusion), I will replace $5 of what was given away (for a couple of reasons explained below*), and he will be allowed to check 5 items off a list we make together to earn back what he gave away (if he wants - it's totally voluntary).
*Ok, first, I told him that I would replace $5 of his $10 because I admitted that it was partly my mistake for not explaining it more clearly about the concept of trading in smaller bills for larger ones...not just being able to give them away. And I totally feel that responsibility.
Second, I told him that I would not replace the entire $10 because it was a gift from an outside family member and it was not proper to ask them to replace something he gave away after it was gifted. (If *I* HAD been the one to give it to him I would still be hesitant about replacing the entire amount, but probably would have anyway.)
Thirdly, I think he does get the lesson surrounding this whole incident, but there was still the question about a natural consequence for breaking a pre-established rule that has been repeated numerous time...about the wallet and money not leaving the house except to go to the store. That's the reason behind the "earning checklist" and another for not simply replacing the full $10.
It's kind of funny, this totally answers a question I was gonna ask several weeks ago: "If a child fully understands the concept of a bank account balance, then is it still important to let him deal with physical paper/coin money?"
Well, yeah, duh. I see that now.
He does totally get the concept of an account balance. I mentioned in a post a while back that he could readily keep track of how much he had to spend when we were at the store, but I never handed him paper money cause I wasn't sure he was ready to track it. This issue even passed through my mind during the while lunch account posts a while back
I guess his great grandparents unknowingly forced me to face this issue by sending him the paper money! Lol.
So, really all that to let ya'll know what finally transpired. It seems like it's gonna work for us...and I'll be sure to let him handle money more...whether he loses it or not. He's gonna have to learn somehow.

Thank you ALL for your perspectives on this.
He and I chatted about this when I picked him up from school and we decided together that it would be fair if we did this: he'll surely get his two $5 from the last two weeks allowance (not a $10, which is my decision to avoid confusion), I will replace $5 of what was given away (for a couple of reasons explained below*), and he will be allowed to check 5 items off a list we make together to earn back what he gave away (if he wants - it's totally voluntary).
*Ok, first, I told him that I would replace $5 of his $10 because I admitted that it was partly my mistake for not explaining it more clearly about the concept of trading in smaller bills for larger ones...not just being able to give them away. And I totally feel that responsibility.
Second, I told him that I would not replace the entire $10 because it was a gift from an outside family member and it was not proper to ask them to replace something he gave away after it was gifted. (If *I* HAD been the one to give it to him I would still be hesitant about replacing the entire amount, but probably would have anyway.)
Thirdly, I think he does get the lesson surrounding this whole incident, but there was still the question about a natural consequence for breaking a pre-established rule that has been repeated numerous time...about the wallet and money not leaving the house except to go to the store. That's the reason behind the "earning checklist" and another for not simply replacing the full $10.
It's kind of funny, this totally answers a question I was gonna ask several weeks ago: "If a child fully understands the concept of a bank account balance, then is it still important to let him deal with physical paper/coin money?"
Well, yeah, duh. I see that now.
He does totally get the concept of an account balance. I mentioned in a post a while back that he could readily keep track of how much he had to spend when we were at the store, but I never handed him paper money cause I wasn't sure he was ready to track it. This issue even passed through my mind during the while lunch account posts a while back
I guess his great grandparents unknowingly forced me to face this issue by sending him the paper money! Lol.
So, really all that to let ya'll know what finally transpired. It seems like it's gonna work for us...and I'll be sure to let him handle money more...whether he loses it or not. He's gonna have to learn somehow.







I would however allow him to do chores around the house to recover his "hamilton". LOL