What if the person gets so busy, he gets another person to help him with the trades in exchange for a fixed amount?

If a person A, who already started a business, hires someone (Person B)to work for a fixed amount, than it is has to be only a share of what that person contributes. Otherwise, if the fixed amount is equal to contributing value of the person B, Person A would not gain anything and would not hire him in the first place. Ergo, from the value the worker, Person B, creates, Person A will take some funds for the development and maintenance of the production (material, rent, taxes etc.), and will keep a portion for himself. At that point, that is exploitation, because the Person B gets only a fraction of what he had earned due to the Person A.

Communists are a huge and diverse ideological group, therefore there are at least two different camps to this. Either you nationalize everything and run the economy from the planned position, therefore (through proxy), turning all companies into public property, or the other (my) camp, in which every worker who works at a business is, after a trial time, given the exactly same share as all other workers and has voting right in how the company is managed. Part of his pay still go for the maintenance and growth, but most goes to him as the owner of his own labor.