Hi Mates,
I am in desperate need here… My present employer has filed in H1b petiion for FY-2014 ( to work for Client A) and its current status is “Post Decision Activity”. In the mean time, I got a good offer from the same Client A to move into their payrolls and to work in UK ( Northern Ireland). Can I leave my present employer and join the Client A before going for H1b Stamping? Can my present employer withdraw my petition which is already in Aproved state?
can someone please advise me here?
Yes, they can withdraw the petition. However, you can still use that petition for a future cap-exempt petition as long as you know the receipt number or have the approval copy.
Thank you Sourabh for your reply… another small question 
Can my employer ask me to pay back expenses incurred for filing H1b petition if i leave the organisation after it is aproved?
It depends on whether you had signed any bond/agreement with your employer while filing H1B petition.
Refer your internal company policies as well, even though bond had not been signed.