I believe that Venmo is part of the PayPal family of services - not related to Square. Not sure of your particular scenario however, if the customer stated they will send you the funds - it's possible you may be out of luck.
If you know the sellers name, or have invoiced the customer in the past, they may have saved their billing information from the past. I also know with Venmo, you can absolutely do a Venmo request which would be absolutely easier.
