In the late 1850s he was heavily involved in promoting and funding the freelance invasion of Nicaragua by William Walker (with the end goal of expanding the slave-holding territory of the United States).

Augusta is the market to which the planters of Upper and Middle Georgia bring their cotton; and if they want to purchase negroes, they step over into Hamburg and do so.

There are two large houses there, with piazzas in front to expose the 'chattels' to the public during the day, and yards in rear of them where they are penned up at night like sheep, so close that they can hardly breathe, with bull-dogs on the outside as sentinels.

[11] According to Parson Brownlow in 1858, Slatter was known as a "rich old bachelor" who owned "the City Hotel, and the New Orleans Arcade, two houses which he rents for about $40,000.

"[18] According to the history Filibusters and Financiers (1916), "on August 28 [1856] a new decree was issued authorizing a loan of $500,000 for twenty years at six per cent, secured by one million acres of [Nicaraguan] public lands.

Messrs. M. Pilcher and S. F. Slatter of New Orleans were constituted agents for negotiating the loan, and arrangements were made for payment of the interest at the Bank of Louisiana.

"[19] There was a Fort Slatter used as a base during the Lockridge Expedition that was located about 20 miles (32 km) "below Castillo" that was named for "a gentleman of New Orleans who had largely contributed to the filibuster cause.

"[12] In July 1857, Slatter accompanied Walker on what was likely a fundraising trip to Richmond, Virginia; the New Orleans True Delta wrote, "It is his intention, we understand, to stop at all the principal intermediate points long enough to kindle the fire of southern patriotism in each place, and we have no doubt the enthusiasm of colonel Slatter will prove a valuable auxiliary to the cause of the General.

"[21] In October 1857, the Louisville Courier reported that "letters from Mississippi state that officers are rapidly recruiting men for Walker's army, destined for Nicaragua.