Regarding senet, I would say you have a legal recourse provided the debtor's name, address, signature and the drawing date are indicated on it. Notarization is not necessary. However, regarding the loan, as a Turkish legal principle, transfer of money through bank is deemed as paying your own debt. Therefore, you must prove that the amounts transferred to him were made under loan. You may rely on the e-mail correspondances to prove that the payments were related with loan but e-mail correspondance is not a conclusive evidence. Furthermore, there is a limitation period of one year for such claims of unjust enrichment in Turkish Law but the judge will not take it into account unless it was raised by the debtor.