So my car was booked on 7th June. Sales tax was due by 15th July (following month) even if we consider the booking date as sale date.
Below is the updated definition from 2008:
TIME OF SUPPLY
Sales tax is chargeable at the time of supply of goods or services. The time of supply means the time of delivery of goods by the supplier.
EXPLANATION
a) Where any goods are supplied by the registered person to an associated person and the goods are not to be removed , the time of supply shall be the time at which the goods are made available to the recipient.
Debit/Credit Notes:
Debit or credit note may be issued to reduce or increase the amount of sales tax payable on a supply if the amount originally charged has changed because of return of goods or for some other valid reason.
Source: https://download1.fbr.gov.pk/Docs/201012211412172902008stax.pdf