"Where the Time Went", a short story by James H. Schmitz, first published in If, November 1968, available at the Internet Archive. The only hardcover appearance known to the ISFDB is in the 1973 anthology Androids, Time Machines and Blue Giraffes edited by Roger Elwood and Vic Ghidalia.

Excerpts:

“I indicated that your problem could be solved, Mr. Belk,” said Dr. Gordon. "And indeed it will be. You see, this situation is so fraught with unethical possibilities that an organization exists which is dedicated to policing the transfer of subjective time among individuals. Such transactions may be quite legitimate. As I explained, a good many people have more time than they know what to do with, they have surplus time which is a nuisance to them. People who need additional time are allowed to draw it from such individuals, providing suitable compensation is made. Since our organization operates with as much secrecy as possible, the donor frequently doesn’t know there has been a transaction. But always he must be compensated. An unexpected stroke of good fortune comes his way; he may find a better job, more suitable to his unenergetic nature, suddenly open to him, and so forth. Both parties have benefited.”

“But why the secrecy?” George asked. “If everybody knew — ” “If everybody were aware of this, Mr. Belk, the situation might get completely out of hand. As I said, the process of extracting time particles from somebody else is very simple, once it is understood. We want no more people to know about it than we can help.”

“I see.” George hesitated. “Then you — this organization — will keep whoever has been stealing my time from doing it again?”

Dr. Gordon smiled. “We can do better than that. Much better. The drainometer recordings indicate that at various periods during the past two days as much as nine out of ten of your time particles have been surreptitiously diverted. This is a blatant crime. The fact that you are, as I previously indicated, inherently somewhat careless with your time has made you an easy victim. But now compensation must be made by those who took advantage of this. When you leave here, you will carry another instrument with you. The next attempt to tap the flow of your time particles will give us a direct line to the perpetrator. In all likelihood we shall find then that you have been preyed upon not by one individual but by a criminal gang.”

“A gang?” George repeated.

“Exactly. As I pointed out, Mr. Belk, time is a commodity. It has value. For some it has great value. Among such people there always will be a number who do not care whether the commodity they want can be obtained legally or ethically, provided only they get it. And there always will be criminal elements willing to supply the commodity for a price. We’re constantly on the lookout for indications of such a situation.”

“And you can make them compensate for what they’ve done?”

“Yes, we can,” said Dr. Gordon. “The organization has very effective means of dealing with such criminals and those who benefit unethically by their crimes. We shall establish exactly how much time was diverted from you and by whom during the past years, and to the last particle this time will be drained from the guilty parties involved and restored. Not in a lump sum, so to speak. But you will have established a time credit with the organization, on which you can draw as your requirements or wishes dictate. In other words, if you should like to operate for a while on the basis of a fully usable forty-eight-hour working day, or even a hundred-hour day, you will be able to do it.”

George was silent a moment. “I hardly know how to thank the organization — and you, sir!” he said then. “There must be some way I can repay you.”

Dr. Gordon cleared his throat. “Well, as a matter of fact, it is customary to charge a fee. The fee goes not to me but to the organization. As I say, time is a commodity. We all can use it. Would a fee of say ten per cent of the time you will regain seem fair to you?” “Eminently fair!” George declared.

He called John Carew next day to tell him of the outcome of the matter.

[. . . .]

“Fine,” said John Carew. “In that case, I’ll look forward to getting a new novel from you within . . . oh, let’s say the next two weeks."

And he got it.