From Missouri the center state: 1821-1915 By Walter Barlow Stevens, page 301
The Assessment of Southern Sympathizers.
In the summer of 1862 there issued from the general commanding at St. Louis an order "to assess and collect without unnecessary delay the sum of five hundred thousand dollars from the secessionists and southern sympathizers" of the city and county of St. Louis. The order stated that the money was to be "used in subsisting, clothing and arming the enrolled militia while in active service, and in providing for the support of the families of such militiamen and United States volunteers as may be destitute." It was extended to other parts of the State.
The unpleasant duty of making and collecting the assessment was imposed upon half a dozen of the best known citizens of St. Louis. The assessment was begun. Collections were enforced by the military. Suddenly the board having the matter in charge suspended the work. The order countermanding the assessment came from Washington. It was terse: "As there seems to be no present military necessity for the enforcement of this assessment, all proceedings under the order will be suspended." v
Two weeks before General Halleck directed discontinuance, a letter was sent to Washington saying "that the 'assessment' now in progress, to be levied upon southern sympathizers and secessionists, is working evil in this community and doing great harm to the Union cause. Among our citizens are all shades of opinion, from that kind of neutrality which is hatred in disguise, through all the grades of lukewarmness, 'sympathy' and hesitating zeal up to the full loyalty which your memorialists claim to possess. To assort and classify them, so as to indicate the dividing line of loyalty and disloyalty, and to establish the rates of payment by those falling below it is a task of great difficulty."
Reviewing the work as far as it had progressed, the writer continued: "The natural consequence has been that many feel themselves deeply aggrieved, not having supposed themselves liable to the suspicion of disloyalty; many escape assessment who, if any, deserve it; and a general feeling of inequality in the rule and ratio of assessments prevails. This was unavoidable, for no two tribunals could agree upon the details of such an assessment either as to the persons or the amounts to be assessed without more complete knowledge of facts than are to be attained from ex parte testimony and current reports."
The writer appealed for a stay of the assessment proceedings. When the letter was written the intention was to have it signed by a number of loyal citizens of St. Louis. But the leading Union men declined to sign. Their feeling against the southern sympathizers was bitter. The war sentiment gripped. Business had been paralyzed. Sentiment rather sustained a policy which proposed to make sympathizers pay heavily toward the war expense. One man, with a deep sense of justice, stood out alone. He had been among the foremost the year previous in counseling the aggressive measures which made St. Louis a Union city. But now, when the Union elements were all powerful, his appeal for fairness toward the minority got no hearing. He signed his letter and sent it to Governor Gamble who forwarded it at once to Washington. Years after the war this letter was printed in a St. Louis newspaper but without the signature and without mention of the name of Rev. Dr. William G. Eliot.
The character of the assessment proceedings will seem almost incredible to this generation. When the board had organized to make the assessment the president addressed a request to "the unconditional Union men of St. Louis" to send in ''such information as they have in their possession which will aid in carrying out the requirements" of the orders. He concluded his request with, "the board wish it to be understood that all communications and evidence will be considered strictly private."
Lion of the valley: St. Louis, Missouri, 1764-1980 By James Neal Primm page 247.
"After the list was completed, Farrar notified those chosen of the amounts due (ranging from fifty to five hundred dollars) and gave them five days to pay. If they did not, enough of their property would be sold at auction to cover their assessments plus 25 percent penalty. Amoung the assessed were Erastus Wells, Alexander Kayser, Mrs. Trusten Polk (wife of ex-U.S. Senator), Dr. William B. McPheeters, Juliette B. Garesche, L.A. Benoist, General Daniel Frost, Mrs. Stephen W. Kearny, Dr. L. C. Boisliniere, and about fifty others...McPheeters charged that Halleck's "Jayhawkers" took his buggy and harness, a rosewood piano, six chairs, two sofas, and a marble-topped table, aggregating $1,110 in value, as payment for his $375 assessment.
I would try newspaper archives. I see references to the Chicago Tribune on lawsuits filed by those who were affected by this law in the later years of the war.