Can the dollar value of a transaction be different at different times? Bitcoin value gets fluctuating frequently. Suppose I am transfering bitcoin to another person who is in another country (like my current bitcoin value is $234.97), i am sending the QR code through e-mail to receive the money , if suppose the person sees my mail after 3 to 4 hrs , will the same bitcoin ($234.97)value be considered or the BTC value which is available when the person receive the money will be considered. Please clarify my doubt.
Amounts in a Bitcoin transaction are set in bitcoin. That is, if you want to receive $5, when the current bitcoin price is $234.97/bitcoin, then you might make a request for 0.0212 bitcoin inside the QR code. 

If the price changes between you making the request and the other person sending bitcoin, then you will still receive 0.0212 bitcoin, but it might be worth $4.50 or $5.50, depending on if the price went up or down. 

However, if you wanted the other person to send you $5 worth of bitcoin at the time that they send it, you could just send the address you want to receive with and a note saying "Please send $5 worth of bitcoin". The other person would have to calculate the current price (and you would have to trust their evaluation, but they can send whatever amount they want).