Valid. According to the "Regulations of the Supreme People's Court on Several Issues Concerning the Application of Law in the Trial of Private Lending Cases", we learned that in private lending contract disputes, lenders often require a third party to sign the loan voucher. The person who signs the loan voucher has four identities: lender, borrower, witness, and guarantor. If a third party signs clearly as a witness, it does not need to bear any responsibility and is valid, so the third party can deposit and withdraw money from the account. The signature on the payment voucher is valid.