(1) A taxing authority may, before accepting any certificate, declaration or document produced before him under these rules, examine the genuineness and correctness of the same and of the contents contained therein and for this purpose he may make such inquiry in relation to, or call for such further evidence in respect of, the agreement of sale or purchase, proof of the receipt, dispatch, transportation, delivery or further disposal, of the goods and, the payments received or made for the sale or purchase of the goods or in relation to anything done in respect of the goods or, the capacity of the seller or the purchaser of the goods or, other relevant matters, as he may consider necessary:
Provided that no certificate, declaration or document produced before a taxing authority shall be rejected before giving the person producing it a reasonable opportunity of being heard.