G. Stanley Hall wrote to Sigmund Freud offering him 1,600 marks ($400) to speak at a conference convened to celebrate the 20th anniversary of the founding of Clark University. Freud declined Hall's invitation, however, when the conference was moved from June to September, Hall wrote to Freud again. The letter read:

My dear Sir:

On December 15th I had the honor of conveying an invitation of our board to you to be present as the celebration which we are planning for next June, which to our great regret you felt obliged to decline on December 29th.

Our situation has changed here in two important respects: first, we have received an addition to our financial budget, which enables us to increase our financial proposition to $750 (3,000 marks); and the date of our celebration has been changed from June to the week beginning September 6th.

In view of these two modifications of our programme, I venture to write you once more in the hope that you may be induced to spend a few days that week with us and give us a few informal lectures, and to allow us to bestow upon you an honorary degree.

I am, with great respect, very truly yours.

Freud accepted and on the 7th September 1909 delivered the first of five lectures at Clark University, during his one and only visit to the United States.

Information via: On This Day in Psychology: A Showcase of Great Pioneers and Defining Moments