The Treasury Department yesterday tentatively cleared Mexican vegatable growers of charges that they hve been selling winter tomatoes and other vegetables in the United States for less than the cost of production.

Florida vegetable growers, who share the winter market about equally with the Mexicans, claimed in a petition filed a year ago that about $200 million worth of the vegetables were being dumped in the United States annually.

Had the Treasury decision gone the other way, Mexican growers might have had to pay special import duties that could have pushed up vegetable prices in supermarkets this winter.

The Florida growers withdrew their original petition asking for relief last July to give the U.S. government time to try to negotiate with the Mexican government on setting limits on low-priced vegetable imports. The negotiators were making progress on an agreement when the petition was refiled Oct. 19, one U.S. negotiator said.The negotiations are still in progress, he said.

Both groups of growers will have an opportunity to present any new evidence or arguments to Treasury before a final determination is made, probably within 90 days.

Treasury was able to sidestep one major issue in the dispute: is it dumping if a perishable agricultural commodity is sold, in effect, in an auction, and on occasion brings less than it cost to produce?

Treasury officials said this happened less than 15 percent of the time. Therefore, there were enough transactions at or above the cost of production that they were able to apply another test of dumping: what prices were charged in sales in a third country?

Under the law, no dumping occurs if prices charged in a third country are similar to those charged in the United States.

In this case, Customs officials chose days at random and checked prices charged in the United States and Canada the same distance from Nogales, Ariz., where the brokers handling the sales are located and where the products usually enter the United States.

Sales of identical products shipped by the same grower on the same day to customers in both Canada and the United States were compared on days randomly selected from the entire growing season in the winter of 1977-78. Treasury officials said.

"From this statistical study it was evident that prices for the same merchandise shipped on the same day to destinations at comparable distances from Nogales . . . were not significantly different," they said.