In the Abraham Lincoln Papers Editors' Preface to the Transcriptions, they point out that "The letters Lincoln wrote to others, where they survive, are mostly to be found either in the papers of the recipients or in special repositories of Lincoln material. Except for occasional drafts and copies of his own letters he retained for his files, the correspondence in Lincoln's papers consists of the letters sent to him beginning at the time he served in Congress in the 1840s, when he first began preserving his papers." So it appears that this could be a very extensive research project, involving contacting many archives to find out if any such letter to Asahel Gridley survives - if it ever existed.
Such a letter certainly does not seem to be reproduced online. Many searches in Lincoln sites such as the ones at Library of Congress American Memory, the Abraham Lincoln Historical Digitization Project (which gets 25 hits for the name Gridley), and the Collected Works of Abraham Lincoln do not appear to shed more light on a letter. Searches such as: "abraham lincoln" gridley, "abraham lincoln" "land ownership", and "abraham lincoln" monopolies get a few letters and quotes that touch on this, but most of them are quoting people such as Black Hawk on land ownership, and a few refer to Browne's book with this quoted passage itself.
