Sears, Roebuck and Co., the nation's largest retailer, has made an agreement with the Credit Union National Association to accept and clear share drafts electronically for its customers.

This marks the first time that either principal has entered into a third-party payment arrangement. And, although the contract is only for a pilot project, its success could pave the way for a fundamental change in retail practices.

Under the program the customer who has a share draft account (interest-bearing checking) at a participating credit union can use it like a check to pay for purchases. An electronic verfification system tells the store immediately if the account contains sufficient funds.

Share drafts for more than the amount of purchase can also be accepted, the change returned. Finally, share draft users may be able to cash them in the stores without having to buy anything, a service now offered to check-writing customers with Sears identification cards.

The advantage to Sears is receipt of payment within 24 hours rather than the several days or a week it now takes checks to clear.

The program was suggested to Sears by CUNA after a 1975 survey revealed that a high percentage of credit union members shop at Sears stores. There are now some 35 million members throughout the country. Of the 22,500 federal and state chartered credit unions, with total assets of $50 billion, more than 750 offer share draft accounts. The number is expected to increase substantially as the relatively new concept of interest-bearing checking accounts becomes more popular.

Present plans call for the Sears pilot project to begin in either Michigan or California in late 1978. Sears is installing optical character recognition readers in its store cash registers in those states. These registers will [WORD ILLEGIBLE] transaction information to the company's central computer.

At the end of the day all transactions will be delivered to the clearing house for credit unions, the only non-electronic step in the process. Within 24 hours the U.S. Central will settle with Sears, and in turn regional centrals and finally the participating credit unions.

Because the project is still in the feasibility state, Sears spokesman Randy Lively declined to state how many stores might be involved or whether Sears would accept other financial institutions along with the credit unions in its system.