No, It is not permitted for a (paper) trader to buy or sell a sustainability declaration for biomethane without the link to therespective amount of physical sustainable material. The transfer of a PoS must always be accompanied by the transfer of ownership of a respective quantity of sustainable material. If system users should find themself with a PoS for biogas / biomethane without the respective quantity of sustainable material at the end of a mass balancing period, it is not possible to transfer those credits into the next mass balance period . Please see the lastest version of ISCC EU System Document 203 “Traceability and Chain of Custody” (chapter 3.4.8).