Two months after signing a long-term debt-refinancing agreement, Venezuela has invoked a clause requiring its foreign creditors to agree to discuss easier repayment terms, President Jaime Lusinchi announced today.

Lusinchi said the provision was invoked "because of difficulties in the oil market." Venezuela relies on oil for 90 percent of its foreign earnings and Venezuelan officials intimated they would reopen the agreement when they signed it in late February.

The contingency clause, unique in Latin American debt agreements, allows Venezuela to ask for easier payment terms in the event of an economic crisis such as the oil price collapse.

Lusinchi did not say what terms he would seek to change in the agreement with 450 creditor banks. But bankers said they would not agree to defer repayment of $750 million in overdue principal payments that Venezuela agreed to make this year. Some government officials have suggested delaying payment of this overdue amount.

"The banks recognize that Venezuela is being buffeted by falling oil prices and that the country needs help," a U.S. banker here said. "But many banks signed the debt agreement only because they were promised that the $750 million would be paid this year, so they'll fight tooth-and-nail against deferment.

The agreement to repay $21.2 billion in public-sector debts to be repaid over the next 12 1/2 years, was signed Feb. 26 in New York.

If Venezuela makes the $750 million payment, bankers said, they are willing to defer principal owed in 1987 and possibly in later years, as well as to provide new loans for trade financing.

Venezuela is also scheduled to make $4.2 billion in interest payments this year.

Venezuela, a leading producer in the Organization of Petroleum Exporting Countries, can afford to pay the $750 million, bankers said, since the country has $16 billion in foreign reserves, easily the highest in Latin America.

But government officials say that the fall in oil prices since last December, from $28-per-barrel on average to about $13 today, has left the oil-dependent nation unable to meet both debt payments and domestic spending needs.

"We have told the banks and the governments of the industrialized countries that Venezuela will honor its legitimate contracted obligations," Lusinchi said. "But we will not do this at the expense of the welfare of the Venezuelan people and the country's economic development."