Sure, I get your point and that would not be a problem for me. If the penance involves making someone else do something you KNOW they will not do then what? I truly believe if I had taken a second or two to think about it, I would have said something during Confession. I also believe he would have given me a different penance. It doesn’t make it any less my fault. Should I just go to Confession again adding the fact I did not do my penance from the previous Confession?
Sounds like an odd, possibly inappropriate, penance to begin with. I think the priest is supposed to give you a penance that you can do yourself and doesn’t depend on others to be completed. It’s not like any of us has the power to
make someone else do anything, really.
Don’t be too hard on yourself - you know there has to be at least some conscious element to sin, either choosing to do wrong or choosing not to care whether you’re doing wrong. Doesn’t sound like there was any of that going on with your confession.
Personally I don’t think you’ve done anything wrong, but it would be desirable to get another penance substituted for the one you can’t do.
Is there any way of talking to the priest outside the confessional and explaining your difficulty? That’d be the best way to handle it, if you can. I believe the priest is allowed, with your permission, to discuss your confession with you outside the confessional, so it doesn’t break the seal of confession.
Of course it may well be just as easy to go back to confession and at least discuss the matter with him. You’re allowed to use the confessional to ask questions and seek advice about sin as well as confessing it (though preferably you should leave it for outside the confessional unless you have a sin or two to confess as well!)