In the Southern USA, you have to remember that those responsible for Jim Crow discrimination were the white capitalist class who collectively were in a position to control banks and fund mortgages.
The economic issue for them was not necessarily the admittance of blacks into the capitalist class, but the maintenance of a system as a whole which was capable of coercing black agricultural workers to work on extremely poor terms (verging on slavery, even after it was officially outlawed).
The terms for blacks had to be poor, because the white farm owners were largely competing to export food to the old world, where there was already abundant land locally, good infrastructure, and white agricultural serfs on much better terms in life.
The relative strength of numbers between white bourgeoisie and black labourers meant that the bourgeoisie proper needed a solid praetorian guard of middle class owners to direct and oversee exploitation and to be willing to lay down their lives militarily in defending the status quo against rebellion.
The bourgeoisie needed also to attract and retain white settlers from the old world who were capable of agricultural management (because of the skills and culture they brought from the old world, and which was not yet endemic in the USA), and they needed a slave class who were deprived of confidence and entitlement to the maximum extent so as to be more inclined to submit.
In the counterfactual where racism did not exist and all were exposed to raw market forces, you would have the prospect of whites becoming immiserated to the point they risked falling economically into the lowest slave class (at which point they would either flee back to the old world, or they would become dangerous radicals rather than genuinely exploitable slaves, and they would frighten and radicalise other whites who feared the same fate), and you would have the prospect of blacks achieving ownership and management and therefore emboldening the rest to aspire to the same and resist their subjugation. You would also, quite simply, risk the whites and blacks uniting together against the bourgeoisie proper.
So for as long as the bourgeoisie of the Southern USA was dependent on the extreme exploitation of black agricultural labour for profit, it was in their interests to insulate whites generally from the risk of falling into a status that could only cause them to be more radical, and insulate blacks from events that could only increase their confidence to resist and rebel against their intense exploitation.
So both very rich whites and middle class whites were broadly in agreement that racism was in their economic interests. Their ability to cooperate to achieve their shared interest doesn't need detailed explanation.
In terms of why a white defector (potentially a rich one) could not simply set up a bank which lent to blacks, it would be because they not only risked conventional violence to their person and property (from well-motivated assailants who potentially had everything to lose), but they also would have faced social ostracism, trade boycotts, and hostile legislation from local lawmakers even, for engaging in activity that would radically undermine capitalist interests if allowed to proceed.
If as a defecting capitalist you wanted to do non-racist business, you'd just move north.