Uncle Sam may be a whiz at spending money, but he is an abject failure at collecting from his debtors, who now owe at least $140 billion, by the reckoning of Sen. James Sasser (D-Tenn.).

The government's shoddy or nonexistent collection habits are costing taxpayers millions of dollars a year in lost interest and bad debts, Sasser reported last week.

Private citizens - man of whom could afford to pay if pressed - and oil and gas companies delinquent in paying lease royalties are, in effect, ripping off the government to the tune of millions, Sasser said.

No one really knows the extent of the problem, but losses due to bad debts alone were estimated at $3.5 billion, and rising, as of last fall.

"This figure for bad debts is up 60 percent in two years," Sasser said. "The net effect is that the honest, hard-working taxpayers are going to eventually be required to pay $3.5 billion more in taxes than they would otherwise be required to pay."

Sasser's figures were produced by the Appropriations subcommittee he chairs and the General Accounting office, which have been prodding the executive branch since last winter to take the situation seriously.

Some of the more startling findings in the ledgers of "disgrace," as Sasser termed it, are:

Many of the $1.2 billion annual oil and gas royalties due the Interior Department are not collected in full because of poor accounting, millions of dollars more are delayed and not interest is assessed.

The U.S. Office of Education has $4 billion in accounts and loans receivable, but one-fourth of it is default - mostly because the office has not dunned student-loan recipients.

The Social Security Administration writes off debts as "hardships" with little investigation. One individual with assets of more than $200,000 had a debt of $1,496 written off as a "hardship."

Veterans Administration educational aid loans were being written off as uncollectable, even though GAO found that many were owed by veterans with good credit ratings, hefty assets and second homes.

"Some of these prosperous and affluent individuals who could well afford to pay their just debts are making a monkey out of the American taxpayer," Sasser said.

"The result is the shift of an enormous additional burden onto the American taxpayer. There is no doubt the executive branch can and must do more on improving government debt-collection operations."

There are signs that the Carter administration has been jarred into action . The Office of Management and Budget is settingup a program to require departments and agencies to more closely monitor accounts and loans receivable.

OMB Director James T. McIntyre Jr., after a meetin with Elmer B. Staats, the comptroller general and head of GAO, last month put the agencies on notice that their accounting procedures must be improved and that OMB will be watching.

Richard Maycock, deputy director of GAO's financial and general management division, said last week that Interior and Treasury have set up financial-monitoring systems.

Maycock said that one of the problems has developed from agencies' practice of writing off debts of $600 or less because collection efforts would cost more than they would recover.

That practice, Sasser said, has made Uncle Sam's debtors even more cavalier about paying back what they owe.

He said GAO auditors found that about 90 percent of a sampling of VA debts was written off after the agency sent its customary three dunning letters and got no response from debtors.

"Some other persons apparently paid their debts down to the $600 limit for referral to the GAO or Department of Justice for collection. They then simply stopped making payments, apparently secure that they would not be prosecuted based on campus newspaper publicity that the government was reluctant to prosecute for an amount under $600," Sasser said.

So far, Sasser has had no luck in getting the executive branch to respond to two of his proposals for dealing more forcefully with delinguents and welshers.

He said that $200 million a year could be collected if the federal government would charge interest on all overdue debts.

And the Internal Revenue Service could help collect billions, he said, by matching computer tapes of tax refunds with tapes of debts and then withholding from the refunds the amount owed to the government.

A GAO study found that one-third of sa sample debt could be recovered by retaining refunds. "It is legal-just as charging interest is legal." said a Sasser aide. "IRS is dragging its feet on this. They claim it would cost them too much."

Such attitudes are part of the problem, added Maycock. "Too many in the agencies in the last 10 or 15 years are more anxious to get money out of Congress than to collect their debts," he said.