The answer to your question is a plain no. It is illegal to demand further monies on a property which was bought at a certain price, but above all it is immoral and unethical. If you are in the position you mention I would suggest you seek legal advice and reject any attempt from the developer to impose a price increase.
I would also be grateful if you could let us know, at your convenience, the details of the developer, as we are actively pursuing these illegal practices. As a law firm we guarantee anonymity and confidentiality.
Follow Up: The developer mentioned is Aifos and the agents are Palmera Properties
It may help us because like you I have not aggreed to any extras. I got fed up of asking PP to sort it out for around 12 months and all they want to do is take your hard earned chash off you and do nothing in return.PP are getting paid large sums of money to act as agents for the buyers and seem to do nothing for it, infact I bet they are making more than Mirador just selling the propertys.
Just a matter of interest I heard a Manager of Mirador say in the background in March when I was in the office on the site and he was stood outside that (THE SILLY BRITS WILL PAY ANYTHING IF WE PUSH THEM) .
I have emailed my soilcitor your message for information