https://help.tallysolutions.com/docs/te9rel64/Voucher_Entry/Advanced_Voucher_Entry/Using_Zero_Valued_Entries.htm

Using Zero-Valued Entries

Zero-valued entries are those entries where a voucher entry is made without any values, that is, one of the parameters (Qty or Rate) may not have a value but needs to updated in the records.

For example , zero-rated VAT or Exempt from Tax Account. You are permitted to make such an entry. The Exempt Ledger account can be viewed for all Exempt Sales.

The items which are given as Free Samples.

Note : Zero Valued entries are allowed only in Sales and Purchase vouchers only and are not available in Receipt, Payment vouchers and journal vouchers.

Enabling Zero Value entry

To enable zero value entries

1. Go to Gateway of Tally > Accounting Features > F2 : Inventory Features .

For example, create a Sales invoice with zero-value entry.

In the above Snap shot you can make out that, if Item A is purchased, then Item B is free.

Even if the values are not mentioned, the quantities of Item B will be reduced from the stocks.