Who is Mr. Jamison,and why does Big Ma want to see him?Mildred D. Taylor's Roll of Thunder, Hear My Cry

After the incident in the store at Strawberry in which the
assertive Cassie objects when the storekeeper Mr. Barnett waits on white people ahead of
her, and she has to apologize to Lillian Jean, tensions begin to mount.  Uncle Hammer,
who is visiting, learns of the incident and angrily jumps into his car with Mr. Morrison
accompanying him. In another incident, Uncle Hammer forces a white man in a truck to
back off the bridge so that he can drive across.  As the white man tips his hat, he
realizes that he has made this gesture to a black man. Mama tells Hammer that he should
not have forced the white man back,

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"But one day we'll have to pay for it.  Believe
me..."

Then, in Chapter 7 of
Roll of Thunder, Hear My Cry, Big Ma, who knows of all the
incidents that have occurred, has talked to Mr. Jamison, the lawyer whose office is in
Strawberry. Big Ma has summoned Mr. Jamison so that she can be sure that nothing will
happen to the property when she is gone; that is, she wants to assure herself that
Harlan Granger will not be able to attain her property on a technicality after her
death. 

Mr. Jamison warns the Logans against using their
land as collateral to back credit for black sharecroppers at the store in Vicksburg
because Harlan Granger will seize any opportunity he has to take away their property. 
But, then, he offers to back the
credit himself.