求一份英文的说服信……You are the secretary of a voluntary organization

求一份英文的说服信……
You are the secretary of a voluntary organization requesting a speaker to come to talk to staff members on the importance of motivation. This person is a leader in his/her field, is much in demand, and would normally expect to be paid. You are unable to offer payment and must make your letter persuasive. 题目是这样的，这个好难不会写，求解答……250字-300字左右！ 求英文高手！！！