Been there, done that...
Was there a written document showing they had agreed to take the units in question back?
This "trade-in" gig (yes, complete with associated "forms and paperwork", essentially a poorly prepared PoA) is certainly not new, nor is subsequent failure to bother to prepare or record a new deed in another name.
The gig is common in Mexico, and seemingly at Cancun in particular (for reasons that I certainly can't and don't pretend to know or understand). Grupo Mayan definitely has had some direct involvement in this gig
there, but GM seems careful not to leave many obvious fingerprints behind.
In essence, the developer assigns grossly inflated figures for the "trade in(s)", supposedly to be applied toward a (...also grossly inflated) developer-direct sale figure.
All of the numbers are completely meaningless and bogus, of course, but "the deal" still manages to "hook" the occasional buyer. In effect, the developer is basically giving the buyer
nothing for the trade-ins. Shady third party entities then get involved and the "trade-ins" can often (usually?) be found being "dumped" on eBay not long thereafter, listed by an obscure and unknown entity, which obviously gets to keep whatever they can get
if they can find a new owner.
I inadvertently benefited directly from one of these shady Mexican gigs just two years ago myself, which is how I learned of all the "inner details" and came to see all of the forms, "trade-in" paperwork and numbers game for myself. I ended up acquiring two very nice back to back Snowbird weeks in coastal SW FL in the same resort / unit, in cooperation with the (U.S. resident) still-owner-of-record, getting new deeds lawfully prepared and recorded in the U.S., directly with the (very grateful) owner of record who thought (incorrectly, as it turned out) that the "trade-ins" were no longer even in his name
at all. I made the trade-in weasels an offer they couldn't refuse;
"You never took or assigned actual ownership, never paid the U.S. maintenance fees due and left the U.S. owner hung out to dry. Your PoA is crap and meaningless here in the U.S. anyhow. A new deed is being prepared and recorded here in the U.S. I'll pay the delinquent maint. fees myself. Take this few hundred $$ as my voluntary thanks for your not obstructing a proper and legal transfer of ownership here in the U.S. Cashing the check overtly signifies your full understanding and complete agreement that you are to now just go away and don't come back". They did --- and didn't, respectively.
I've enjoyed those two consecutive Snowbird weeks in SW FL in the same unit a lot, each year since.
P.S. Cutting the Mexican "trade-in" parasites right out of the ownership transfer picture and process was relatively easily in
that particular instance, because the owner of record, the "traded-in" weeks and the new owner (me) were
all right here in the U.S. It would not be as easy to "steamroll" the parasites if the "traded-in" weeks were in Mexico, where the "new" (developer-direct, overpriced) purchase was conducted.