Why does it show them arguing about TNE vs. TNA?

The four men are acting like children. They're trying to make each other laugh, because dad is so angry and they're supposed to be taking the situation seriously.

Donnie says "T&A" (which is slang for t*ts and a**) with the intention of making his friends laugh, knowing that he can cover it up by saying that he actually said T&E (which is a business term that means travel & expenses).

The scene was included overall to show that the men think they can do anything they want without getting in trouble. The "T&A" joke included just as another example of the men acting immature, and laughing when they should be taking something seriously.

He says T&A because they spent the money on strippers and hookers. I mean escorts, like the real life person whom the film is based on was known to do. He literally means he spent it on Tits and Ass. The father knows that EJ entertainment is some sort of escorts service. Its a flippant reply to Max, who as their accountant is already mad about the mess they are putting in his hands. And Donnie continues trying to bullshit his way through it. They argue because Max cares and Donnie doesn't.

It is shown that way to illustrate to the audience how ridiculous and irresponsible they are with money, squandering it away. They don't care cause they know they can get away with it and someone like Max will fix it. It's just another sequence building up the characters as the immature jackasses that they are. And because the movie is a slightly dramatized and exaggerated retelling of an autobiography, it's likely that a similar conversation happened.

He said it wrong, making fun of the intense situation. He said T&A but the correct therm involving tax deduction, what they sounded to mean, travel and expense, or travel and entertainment expenses.

i got this from a good website:

The term T&E means either 'Travel & Expense' or 'Travel & Entertainment Expenses'. These phrases (T&E, T and E, travel and expense, and travel and entertainment) are often used when talking about the second largest operational cost, after salaries. According to Aberdeen, "business expenses related to travel and entertainment encompass 8% to 12% of the average organisation's total budget".[1] So it's a big deal if you're concerned with either saving costs or making your business processes more efficient.