Structure question (entry in entry?)
Craft has a pretty extensive relationship system, which is what you'd probably want to take advantage of here.

You could make another channel called "Offers" and each offer would be an entry in that section.

Then you could create an entries field called "Ad" and save it in your "Offers" section's field layout and have its source set to "Ad".

Then, when saving an offer, you'd select the ad(s) in the ad field the offer belongs.

You'd be able to query those relationships from the template side like so: https://craftcms.com/docs/entries-fields#examples