I'm extremely sorry to inform you that i won't be able to attend your birthday. It's with my greatest misery that I tell you this. I however regret this and wish to make my presence possible for your birthday party. I hope you will forgive me for this.  :(
Hi friend ,
first of all happy birthday
and i am sorry that due to some circumstances i am not able to prsent your gift since this time my financial condition is not good.
i assure you as my financial status will improve i will surely buy a beautiful present for u .
hope you understand
yours lovingly 
(it can be your name )