You are the one who don't understand what 1 to 1 exchange means. It simply means the seller exchanging the faulty product with another new unit ON THE SPOT.

Going by your logic of:
What you are doing now is:

"Ehhh abang, sorry if your drink is not nice. Not our fault okay? I give you 1 to 1 exchange for a brand new cup, but only after I call up Milo Malaysia to exchange that pack of Milo, next month okay?"

Do you even know how ridiculous you sound?

Just stop it man. Game's over. You're just making yourself look ridiculous and the more you try to deny, the worse it becomes.

Here's my advice: Just play it smart. Apologize and refund/exchange the buyer's unit immediately and absorb the cost instead of not settling this matter as it would only damage your reputation further.