Workers who received President Carter's new "real wage insurance" tax credit -- designed to protect employes who agreed to his new 7 percent wage guideline from more-rapid inflation -- will have to pay taxes on any rebate they receive, according to key officials.

Although the measure is intended to make up for lost purchasing power if inflation exceeds 7 percent, the credit will be considered part of workers' earnings, and thus will be subject to federal incometax, the White House will propose. However, it will not be subject to payroll taxes.

The White House proposal, approved by Carter, but not yet sent to Congress, calls, as reported earlier, for a tax credit of up to 1 percent of a worker's first $20,000 in wages for each percentage-point that inflation exceeds the guideline -- a maximum of $600 if prices rise by 10 percent.

However, officials said the plan also contains these provisions not cited in earlier descriptions by Carter lieutenants:

The tax credit will be "refundable." That is, in cases where a worker does not earn enough to benefit from a tax credit, the government will send him the rebate as a direct cash payment -- much as now is done with the "earned income credit" for the poor.

Managerial and supervisory workers will have to meet an extra test to qualify for the credit: They'll have to include under the 7 percent limit both their wages and 25 percent of any bounses or irregular payments they receive. For rank-and-file workers, it's only wages alone.

The administration is including a "small business exemption' that would allow firms which employ 50 or fewer workers to choose not to certify their employes as eligible for the tax credit -- provided they formally notify them in advance.

Part-time workers and farm workers will be eligible for the tax credit, provided their wages have not risen more than 7 percent during the period. However, self-employed persons will be excluded. Low-wage workers would be eligible if their wages rose less than 7 percent.

Administration estimates show the program would cost between $5 billion and $15 billion a year, depending upon whether inflation remains at 8 percent or climbs to 10 percent or higher. If prices rose only 7.5 percent as Carter predicts, the tab in fiscal 1980 would be $2.7 billion.

The proposal to make the wage-insurance tax credit taxable is being defended by officials as reasonable from an economic viewpoint. Officials say if workers had received a larger pay hike instead, that money would be subject to regular income taxes.

However, outsiders speculated the move might dampen support from organized labor, which has not been visibly enthusiastic about the wage-insurance proposal. Labor's support could prove crucial to the measure's passage. It was as a sweetner for unions that the plan was proposed.

Both the House Ways and Means Committee and the Senate Finance Committee have been cool to the proposal. Ways and Means chairman Al Ullman (D-Ore.) has pledged to hold the hearings on the measure, but has been wary that it might become a vehicle for bigger tax cuts.

As Carter promised earlier, the plan would cover only those wages earned 1979. However, the new proposal would give Carter the authority to extend the plan through 1980 merely by executive order. Congress could block it by resolution, but Carter could veto that as well.

The proposal also would give the president authority to change the limits of the program. The plan now would cover workers in cases where inflation rises to between a 7 and 10 percent pact. Presumably, Carter could reduce this range somewhat if he thought the situation had improved.

The measure contains a complex plan for deciding whether wage increases meet the administration's guidelines. The pay-hike would be measured from the third quarter of 1979, with a September 1-to-August 31 period for managerial workers.

The administration would count the inflation rate for the year by measuring the rise in the consumer price index between October-November 1978 and October-November 1979. Taxpayers would claim the credit on their regular 1040 or 1040-A forms, where an extra line would be provided.

The administration is expected to make the plan public officially early next week. A Treasury task force has been working on the proposal since Carter unveiled his wage-price guidelines program in October. The plan is base don a proposal by Johnson administration economist Arthur M. Okun.