Venmo is an app used to send cash with a little note to describe what it’s for. It’s owned by PayPal. When someone asks for someone else’s Venmo, it normally means they want to send cash. The person with $1 remaining must have given their Venmo info out hoping to get some help, but instead the person who asked for it requested their last dollar.
That’s an explanation of what is happening here. I can’t tell you why it’s funny as requested, but that’s because it’s really not that funny