If we assume good faith, then this person acts as a representative of the company which the OP has a relationship with. Neither did the person offer something "for nothing", he offered a slight discount which may very well be related to a fare drop, a POS differential, or a private fare that only UA sales reps have access to.

If you show interest in buying a product, leave contact details, and you get an unsolicited call or email from a sales rep, it can definitely be qualified as annoying, but I wouldn't say it is a scam.

As indicated above, it's not an offer for nothing, it's an offer which requires OP to pay. And I wouldn't call it an obvious "too good to be true"-offer either.

While I agree it may look strange to us for a rep to ask details that way, it may be an acceptable way in the culture of MX, and in itself is insufficient to qualify as scam (this seems to be prejudice at work). As a general guideline, however, credit card data should be guarded, and OP should be able to find an acceptable way of reconciling this with the offer of the company he received, with a slightly discounted ticket and a rep recording a sale as a result, and no one scammed out of any money.