The Venmo line describes a supposed generation gap among employees.
If a young co-worker offers to pay you back for lunch with Venmo, and you hesitate, you’ll immediately be declared “above the Venmo line,” and might as well be demanding payment with a Diners Club card.
Venmo is a payment app owned by PayPal that’s virtually ubiquitous among 20-somethings and virtually invisible to anyone older. Hence, the Venmo line. (Source)