Least suspicious Civil War expenditure

Thurlow Weed was the political boss of NYC during the Civil War. In 1863, he was summoned to the White House:

Weed undertook some special missions for President Lincoln. One was a peculiar request in February 1863 that Weed raise $15,000 for an unspecified purpose. The most curious illustration of Lincoln’s confidence, an incident that is still cloaked in mystery, came at the height of the war. On February 18, 1863, Weed received a telegram from Lincoln’s secretary, Nicolay, asking him to be in Washington by the following morning. The summons was obeyed and, after breakfasting with Seward went to the White House.

Lincoln explained that there was urgent need for $15,000 and that this money could not be taken from any available appropriation. Could Weed raise the amount? Weed could and did, but he never found out the reason the money was needed. Weed requested a letter of explanation which Mr. Lincoln furnished: “The matters I spoke to you about are important. I hope you will not neglect them. Truly yours, A. Lincoln.”

There's some suspicion it may have been for bribery around local elections in Connecticut to avoid risks to arms manufacturing.