Vice President George Bush today unfroze some aid to near-bankrupt Sudan and signed an agreement to provide $15 million for fertilizers and insecticides for cotton cultivation, the country's biggest foreign exchange earner.

Speaking at a news conference at the end of a three-day visit to Africa's largest country, Bush said the agreement showed a "certain flexibility in our approach." The vice president then flew on to Niamey, the capital of Niger, the second stop on his three-nation African tour.

There was no indication that the United States was relenting on the remaining $201 million in earmarked, but frozen economic support aid to Sudan for fiscal years 1984 and 1985.

Informed sources said a test of wills apparently is continuing, with beleaguered President Jaafar Nimeri still unwilling to carry out key economic reforms in return for resumption of aid from the United States and other foreign donors.

If anything, the United States blinked in the immediate confrontation, according to financial specialists. They reason that Nimeri gave nothing away and outwaited the Reagan administration, gambling that the desperately needed fertilizer and insecticides had to be provided now or the year's cotton crop would suffer.

This time pressure apparently prompted the United States to relent, the specialist said.

One knowledgeable source, who requested anonymity, said the crisis point "could come within 10 days. Either the United States sticks to its guns and makes Nimeri take his terrible medicine or it decides to relent out of fear that the patient will succumb."

In any case there was nothing to suggest that a rapid breakthrough was in the offing after Sudanese Finance Minister Abdel Rahman Wahab conferred in Washington last week with officials of the International Monetary Fund on a package of Sudanese proposals to end the crisis.

U.S. officials had pinned their hopes on these talks, arguing that a nod from the IMF would be enough to allow the United States, Britain, West Germany, Saudi Arabia and other donors to resume aid.

But with the Sudan $110 million in arrears to the IMF, the Washington-based financial institution cannot even negotiate formally with Nimeri's government.

Behind such diplomatic maneuvering lies a record of broken promises to international financial institutions and donors that has destroyed once high hopes of transforming Sudan into the breadbasket of Africa by developing its oil resources with western know-how.

The economic mess reached such proportions last October that the administration decided the United States alone should not attempt to bail Nimeri out.

But foreign exchange has become so tight that those industries that have survived are threatened with closure within a month, raising the specter of throwing thousands out of work.

Financial sources said the litmus test is whether the United States releases $80 million earmarked for oil over the past and current fiscal years.

If released, those funds could free foreign exchange for other vital requirements.

But the enduring oil shortage has a crippling multiplier effect on the rest of the economy.