"The Accountant", a short story by Robert Sheckley, first published in The Magazine of Fantasy and Science Fiction, July 1954, available at the Internet Archive. Does any of these covers ring a bell?
The child of a respected sorcerer family scandalizes them by choosing a career as an accountant.
Morton looked uncomfortable and scuffed his feet on the floor.
"You know, not many boys have a chance to become wizards in this day and age."
"Yes sir, I know." Morton looked away abruptly. In a high, nervous voice he said, "But Dad, I want to be an accountant. I really do, Dad?"
Mr. Dee shook his head. "Morton, there's always been a wizard in our family. For eighteen hundred years, the Dees have been famous in supernatural circles."
Morton continued to look out the window and scuff his feet.
"You wouldn't want to disappoint me, would you, son?" Dee smiled sadly. "You know, anyone can be an accountant. But only a chosen few can master the Black Arts."
The parents summon some sort of demon or scary relative to subdue the rebellious child,
The scary relative was the boy's dead grandfather:
Morton grinned, and turned back to his books.
Mr. Dee was unable to stand it any longer. "Damn it," he shouted, "aren't you scared?"
"Why should I be?" Morton asked. "It's only grandpa."
After Grandpa Dee fails to make an impression, Mr. Dee summons a real demon:
"That does it, Mr. Dee said. "The boy is too cocksure of himself. We must call up Boarbas."
"No!" his wife said.
"I just don't know any more," Mrs. Dee said, on the verge of tears. You know what Boarbas does to children. They're never the same afterwards."
"Mr. Dee's face was hard as granite. "I know. It can't be helped."
"He's so young!" Mrs. Dee wailed. "It—it will be traumatic!"
"If so, we will use all the resources of modern psychology to heal him," Mr. Dee said soothingly. "He will have the best psychoanalysts money can buy. But the boy must be a wizard!"
"Go ahead then," Mrs. Dee said, crying openly. "But please don't ask me to assist you."
How like a woman, Dee thought. Always turning into jelly at the moment when firmness was indicated. With a heavy heart, he made the preparations for calling up Boarbas, Demon of Children.
but the child is ultimately victorious by casting the "seven words of invalidity(?)": dissolution, annulment, expiry, forfeiture, termination [...] and death."
The demons great horned hands reached for Morton; but quickly the boy opened the accounting book. "Save me!" he screamed.
In that instant, a tall, terribly thin old man appeared, covered with worn pen points and ledger sheets, his eyes two empty zeroes.
"Zico Pico Reel!" chanted Boarbas, turning to grapple with the newcomer. But the thin old man laughed, and said, "A contract of corporation which is ultra vires is not voidable only, but utterly void."
At these words, Boarbas was flung back, breaking a chair as he fell. He scrambled to his feet, his skin glowing red-hot with rage, and intoned the Demoniac Master-Spell: "VRAY, HAT, HO!"
But the thin old man shielded Morton with his body, and cried the words of Dissolution: "Expiration, Repeal, Occurrence, Surrender, Abandonment and Death!"
Boarbas squeaked in agony. Hastily he backed away, fumbling in the air until he found The Opening. He jumped through this, and was gone.
The tall, thin old man turned to Mr. and Mrs. Dee, cowering in a corner of the living room, and said, “Know that I am The Accountant. And Know, Moreover, that this Child has signed a Compact with Me, to enter My Apprenticeship and be My Servant. And in return for Services Rendered, I, THE ACCOUNTANT, am teaching him the Damnation of Souls, by means of
ensnaring them in a cursed web of Figures, Forms, Torts and Reprisals. And behold, this is My Mark upon him!"
The Accountant held up Morton's right hand, and showed the ink smudge on the third finger.
He turned to Morton, and in a softer voice said, "Tomorrow, lad, we will consider some aspects of Income Tax Evasion as a Path to Damnation."