Where would you find this opinion?

[David Lockmiller]

Diplomatic recognition of Haiti and Liberia had long been resisted on the grounds that those nations might send blacks to represent them at Washington. Lincoln, however, did not object to that possibility. When James Redpath told him that President Fabre Nicolas Geffrard of Haiti was willing to appoint a white representative rather than a black one to Washington, Lincoln replied: "Well -- you can tell Mr. Geffrard that I shan't tear my shirt if he does send a negro here!" [footnote 136, citing as reference Washington correspondence by Van [D. W. Bartlett], 24 June, Springfield, Massachusetts, Republican, 27 June 1862.] (The Haitian government appointed a black army colonel, Ernest Roumain, as its first minister to the United States.)

Abraham Lincoln: A Life, Vol. Two, p. 351, by Professor Michael Burlingame.

I presume that "I shan't tear my shirt" may be a reference to Robert Burns' poem, but I do not know.