It is with no small regret that——
I am exceedingly sorry that your request comes to me at a time when I am so pressed by my own affairs, that I cannot, with any convenience, comply with it
Pardon my indiscretion
(Conventional——Assistance)
I shall be very happy to assist you
I have had great pleasure in being useful to you
Permit me to serve you in any way I can
May I give you some assistance
If you can assist me I shall take it as an extraordinary kindness