By Rabbi Chaim Chazzan
May a publisher offer a discounted price to those who pay in advance before the sefer is printed?
The Alter Rebbe rules that one who sells an object at a discounted price for advance payment transgresses ribis midrabanan (avak ribis).
The reason for this: The advance payment is seen as a loan. The receiver of the loan (the publisher) then repays the loan with goods (seforim) worth more than the amount paid because of the money laid out by the buyer. This is similar to ribis.
Under certain conditions Chazal permitted such a transaction: (1) If the seller has the item in stock. Then it is considered as if the buyer received the item at the time of payment (though he collected it at a later date) or (2) If the article on sale doesn’t (and won’t) have a fixed price. Then it’s not noticeable that a discount was given due to the advance payment. However in our scenario neither of these conditions exist.
Yet, some acharonim allow this type of sale specifically for the purpose of printing a sefer. Since spreading Torah is a mitzvah and we find Chazal allowed ribis midrabanan for Shabbos needs. However, some recent acharonim contend that one should not rely on this heterfor printing seforim. Since it is often done for financial interests it may not have the same mitzvah status as seudas Shabbos.
It is therefore good that a heter iska be made, which arranges for the publisher and the buyer to enter a joint venture, with the return for the buyer being the value of the seforim. Another option is that for a while after print the seforim should be sold for the discounted rate, so that the advance payers received nothing extra for their early payment. 
 שוע”ר הל’ ריבית סכ”ג
 רמ”א יו”ד קע”ג ס”ז (דלא כש”ך וט”ז שם).
 דרכ”ת יור”ד סי’ קס ס”ק קמח בשם שו”ת מעשה אברהם
 אדה”ז באו”ח סי’ רמ”ב ס”ט (ודלא כמשנ”ב שם).
 שו”ת מנח”י ח”ד סי’ צ”ט. וראה בספר תו”ר פרק י’ הערה קי”ב, ובס’ מלוה ה’ ח”א עמוד תכ”ט
 מסורת משה עמודים רמ”א-ב’
 ראה מלוה ה’ שם