American University basketball player Manuel Nadal has forfeited a year of eligibility because he "received improper expense payments" while playing in an unsanctioned summer league in the Dominican Republic, an NCAA spokesman said yesterday.

The ruling, made after AU Coach Ed Tapscott brought the case to the NCAA's attention, leaves Nadal with three years of eligibility. Tapscott said Nadal, who has not played at all during the 1985-86 season, would count this season as the one forfeited and would be eligible to play next season.

Nadal, a redshirt freshman, was unavailable for comment.

The ruling also might signal more difficulties for Tito Horford, the 7-foot center who is Nadal's boyhood friend and was his teammate on the summer league team, called Club Naco, which plays in Santo Domingo. Horford originally signed a letter of intent at Houston, then enrolled at Louisiana State and recently transferred to the University of Miami.

"There could be other players affected by this legislation," said Doug Johnson, a director of legislative services with the NCAA. "We're seeking to apply the rule evenly to all student athletes and all institutions, not just the ones who report the problem themselves."

Johnson would not comment specifically on Horford because of the ongoing investigation.

Nadal, according to Tapscott and Johnson, received $1,100 for expenses while playing for the team last summer. Johnson said the amount was more than the NCAA deemed appropriate.

"The $1,100 was for a little over two months," Tapscott said, "and was for transportation, food and lodging. It works out to about $16 a day, a piddling sum."

Tapscott said Nadal had been told that if he didn't play in the league, he wouldn't be eligible to play for the Dominican Republic's national team.

"There is an (NCAA) exception for players from other countries who are trying out for their national team," Tapscott said. "And this league is a preview for that, and they told him that if he didn't play, then he wouldn't be able to play for the national team. But they (the NCAA) didn't find that argument compelling.

"He had some awareness (of a possible rule violation), but he was told that if he wants to play on the national team and in the Olympics, then he's got to play. His choice was between a rock and a hard place."

One section of the NCAA constitution dealing with payment of expenses, says, " . . . Expenses received from an outside amateur sports team or organization in excess of actual and necessary travel and meal expenses for practice and game competition shall be considered pay."

"We can't create exceptions to existing legislation," Johnson said. "We can apply what exceptions do exist."

Tapscott said that one problem involves the varying definitions of amateurism.

"He's an amateur by Dominican standards," Tapscott said of Nadal. "The rules of amateur status differ from country to country. And it was because of the differing rules the eligibility question came up, and his eligibility was affected by his participation in the Dominican summer league."

Because Nadal was told he had to participate, Tapscott said he thinks the situation is similar to that of athletes taking part in the National Sports Festival in this country.

"We need to make our rules more in line with other countries," Tapscott. "The players who are in that (sports festival) have their transportation, lodging and food paid for by the Olympic Committee, which is what Nadal got."