In BUSY, you will be able to save te voucher with zero amount. Further, you can restrict for same by follow these steps:
Step 1: Go to administration section then click on configuration
Step 2: Choose the voucher series configuration then click on voucher series group tab
Step 3: Now, select the required voucher series group and speciFinancial Year Y to enable zero value validation option then click on configure tab
Step 4: Tick on disallow zero voucher amount and save the configuration. Afterward, you will be restricted to create voucher with zero amount.