BOSTON, Feb. 24--State Senate President Maurice A. Donahue (D-Holyoke) succeeded today in weakening Governor John A. Volpe's $135 million sales tax bill.

Donahue, who has led the fight against the sales tax for a year and a half, offered an amendment to the bill which would grant households an annual $10 tax credit for each dependent child. The amendment was adopted 22 to 17.

Under Volpe's original bill the head of each household earning $4000 a year or less would have received an annual $4 tax credit for himself and each of his dependents.

Volpe said last night that the $10 credit would cost the state $18 million, and some of his advisors put the figure at $20 million. Donahue said his amendment would cost only $8 million.

But Senator John J. Conte (D-Worcester) said he will propose another amendment restricting the $10 credit to those families earning less than $4000 a year.

Emergency Bill

WASHINGTON, Feb. 24--The House of Representatives overwhelmingly endorsed and sent to the Senate a $415 million emergency program to boister the economy of South Vietnam and other nations.

The bill was sent to the Senate on a roll call vote of 350 to 27. None of the funds authorized would be used for military purposes.

Of the total appropriation, $275 million would be used for economic support of South Vietnam and for community development, refugee relief and restoration of war-damaged villages.

Twenty-five million dollars would be used for economic aid in the Dominican Republic. Aid programs in Thailand and Laos would each receive $7.5 million. The President's contingency fund for unforeseen emergencies would receive an extra $100 million.

Leaders of both parties called for approval of the bill. There was no criticism of the administration's Vietnam policy during the four-hour House debate.