Theres is no such limit as a general rule.
Various rumors have been around like this and I even read once in a travel book you could NOT change your money back, and another that said you can only change back what you could prove you started with. All hogwash.
Various cash exchanges however may have a policy of not allowing you to buy dollars unless you can prove you started with dollars and exchanged them with that particular exchange. Otherwise, how would the cash exchange themself ever be able to buy your dollars, since they didn't start with dollars.
You can even sell your pesos for US Dollars in Miami at the local cash exchange at the airport (for a 20% loss on the exchange rate of course), and I guarantee you they will not enquire if it is more than 30% of the pesos you started with.
There is no 30% tax for transferring money either. That may be what one bank charges, but I've wired money and only paid wire charges. If there was such a charge, people would just walk over to Western Union.
By the way, that's another way to convert pesos to dollars. One can elect to have the recipient receive their money in the local currency of the recipient.