Your defense should be on the following lines:
1) Since you are a trading firm, in the normal course of business, you will only supply material that the customer wants.
2) In the sense that you do not recommend the usage of products(rubber pipe) in lieu of Metal pipe as it is not in the nature of course of your business to sell your products on Design and Installation basis.
3) In accordance to law of contract, it is the responsibility of the buyer (Caveat Emptor), to be fully conscious of the product that he is intending to buy from you and put in to the correct use of the product. If the customer has put in to use the rubber pipe in place of metal pipe, then he is at fault for the fact that the customer though being fully aware that the metal pipe has to be used, has opted for a rubber pipe instead.
4) Also since you are a trading firm, you only buy and sell goods and it is the duty of the manufacturer(rubber pipe manufacturer) if at all there is any manufacturing defect in the rubber pipe.
5) From what you said, you or your firm will not be liable for the fault of your buyer and hence no worries.
6) Your case as stated by you, does not amount to cheating in any which way and does not constitute a criminal dispute.
7) From your statement, it is a purely civil dispute, actually police cannot question you and only the courts have the authority.
Next course of action:
Technically you do not need to appear in person at police station and instead you can ask a lawyer to represent you for and on behalf of you and your firm and can submit your statement in writing.
Hope this helps.