This article offers the 'editio princeps' of a fragmentary Greek papyrus, which was discovered among the unpublished documents of the Department of Papyri of the Austrian National Library in the course of the Project "Papyri of the Early Arab Period Online", funded by the Andrew W. Mellon Foundation. The text reveals a rare type of legal contract: a ‘cautio indemnitatis’, i.e. a security for a guarantor. The background of this business was probably a deed of surety. The document is addressed to a well-known member of the social and economic elite of Byzantine Egypt on the eve of the Sassanian and Arab conquest.