Payment-by-cheque requirement for deductible business expenses: non-cheque payments may be disallowed unless exceptions apply, including proof provisions. Section 40A(3) conditions deduction on payment by crossed bank cheque or crossed bank draft for deductible business expenditures exceeding the monetary threshold; Rule 6DD specifies exclusions (pre existing cash contracts, book adjustments, payments in banking less localities, payments to agricultural and cottage producers, certain banking instruments, payments to financial institutions, and specified terminal benefits) and a residuary exception for exceptional unavoidable circumstances where the assessee proves genuineness and payee identity to the income tax officer.