It is unclear from description (and I am too lazy to read the code) how it works.

The first question is whether such paper "bill" would guarantee payment. Could issuer refuse to pay it, when presented. In other words such paper bill is analogous to dollar bill or bank check? If this feature could not be implemented with Venmo API, perhaps one can build some kind of escrow service.

My second question is about privacy. Is identity of the original paper bill "issuer" is disclosed to recipient?