Updating since it’s been a while. Currently Square Cash does not have an API that we can use to integrate. At this point we would need Square Cash to change in order for us to do the integration.Leona Dorothy commented
Even if it didn't fully integrate, it would be great to be able to enter in a Square Cash id in your profile and cause that to display a button for it in the payment options which would link to the user's cash.me page (based on the user id). That would be simple enough. Would let people know if the option is there for someone they're paying back -- even if the amount wouldn't auto-populate or link back. Could have a qualifier or help info that says you have to record as a cash payement after sending money through Square Cash.