The Tory leader was entitled to the bonus after losing his post as a Cabinet minister in the Labour landslide on 1 May.
During a visit to a racing circuit at Snetterton, Norfolk, he told journalists that he had returned the payment.
"I have given pounds 15,000 back to the Government without trumpeting it, without making a special announcement about it, because I thought I was being paid too much," he said.
However, since winning the leadership of the Tory party, Mr Hague has taken his full entitlement as Leader of the Opposition, amounting to a total pounds 98,000. He added: "I have committed myself to inflation-linked increases."
Mr Hague's self-sacrifice failed to impress Downing Street officials, who pointed out that as the Leader of the Opposition, Tony Blair had declined three increases in salary and took a reduced salary. "William Hague's salary is pounds 12,000 more than than Mr Blair as Leader of the Opposition," said a spokesman, who suggested Mr Hague would take a cut if he accepted performance-related pay.
The Conservative leader also took a month to decide to hand back the money to the Treasury. Tory sources said he received the pounds 15,000 in late May or early June, and decided to return it in July, after he had won the leadership.
Mr Blair plunged the Government into a dilemma over pay rises when he announced last week that he would forgo a further rise next April of pounds 40,000 to show a lead to 1.3 million public sector workers, including nurses, doctors and teachers, who were being asked to show pay restraint.Reuse content