United Mobiles,Mobile Zone,Chimera,infact,all agents selling devices here in Pakistan are thieves,blackmailers,cheaters and looters.Don't go to them,rather I'll suggest you to visit Hafeez Centre,if you are in Lahore.They'll have a much better option for you.
Which application did you install,after which it started giving problem ? Well,personally I don't think that by just installing any application 'Ram' of the device will go corrupt.Ram is the memory of the phone,we all keep on installing many different applications often,but never seen that it goes corrupt.Even flash different Roms,if a Rom flash goes bad or a wrong Rom is flashed,we can always reflash another Rom to get back the phone in working status,BUT RAM,never,They are just making a false story to extort money from you.Yes ! RAM itself can fake out,or phone do get dead due to some Hardware failure,thats different.
But for a price of Rs.20,000 just to replace the main board,than I'll suggest you just keep it like this and may someday you get a better chance to get it repaired,or sell it at the best offer you can grab and buy another,I have seen a couple of Jag3 Pda's at Hafeez Centre for less than 20,000 although, slightly used,but still you wud be better off ,or buy a wizard,I-mate k-jam,T-mobile or O2 for much lesser price.