President Carter's plan to sell 50 F5 jet fighters to Egypt had been delayed because Saudi Arabia, which is supposed to finance the deal, has held up payment over questions about the price, administration and diplomatic sources said yesterday.

The first batch of F5s was supposed to have been delivered to Egypt no later than last month, but neither the plane nor its parts have seen shipped, the Pentagon confirmed yesterday.

Although Pentagon spokesmen declined to explain why the deliveries have been held up, government officials said the Saudi money Egypt needs to finance the $600 million deal has not been forthcoming.

Diplomatic sources said that the Saudi government is demanding that Washington explain why teh F5s destined for Egypt will cost so much more than the same planes previously purchased by Saudi Arabia.

These sources stressed that the Saudis are not attempting to embarrass Egyptian President Anwar Sadat for his negotiating a separate peace with Israel. Rather, it is a case of the Saudis trying to make sure that they get their money's worth, the sources said.

These diplomatic sources added that the Saudis probably will come through with the money, probably by the end of next, if the Defense Department clears up the financial question. The Saudis are so eager to get the 60 U.S. Air Force F15 fighters that Carter had promised to sell them as part of the package, that they are unlikely to shoot down the deal for the F5s.

Administration officials this spring persuaded Congress that Egypt be allowed to buy the F5s to modernize its air force in order to feel secure enough to negotiate a peace treaty with Israel. That argument is seen by administration officials as even more compelling now that Egypt and Israeli officials are in the final stages of negotiating a treaty.

The Carter Mideast package calls for selling Egypt eight F5Fs two-sent trainer versions of the fighter - and 42 F5Es, capable of aerial combat and bombing within a limited range.

Israel, under that same deal, will get 25 F15 fighters and 75 F16s. Both planes are considered hotter than the F5Es destined for Egypt.

The egyptian air force is considered to be in poor shape by U.S. military officials.Although the Soviet Union supplied Egypt with hundreds of Mig jet fighters, relatively few of them are flying for want of spare parts.

In another arms sales development, the Carter administration has decided to shelve for now a plan to upgrade the F5 fighter and sell it to Nationalist China, which also wants to modernize its air force.

The proposed F5 for Taiwan, called the F5G, would carry a bigger air defense missile than the F5s already in the Nationalist China's inventory. The F5G, however, would have to undergo several major changes, including substituting for the two smaller ones now propelling the F5E fighter planes.