Mr Modi's fresh allegation against Lalu Prasad's family, which the RJD termed "baseless", came a day after he urged the Election Commission to disqualify Tej Pratap as a legislator for "concealing" details of assets in his election affidavit.
The BJP leader claimed that on March 23, 1992, then state minister Brij Bihari Prasad's wife Rama Devi gifted two plots - one measuring 9 acres 24 decimal and the other 3 acres 88 decimal - in the Kudhni area of Muzaffarpur district to Tej Pratap as a reward for "taking care" of her.
Rama Devi is at present the BJP MP from Sheohar. Stating that Tej Pratap was only a three-year-old child in 1992, Mr Modi said, "The gift deed mentioned that Rama Devi gifted the land to Tej Pratap out of sheer love and affection, and because she was impressed at the way he had "taken care" of her.
How could a three-year-and-eight-month-old child take care of or look after Rama Devi, asked the Leader of the Opposition in the Bihar legislative council.
Mr Modi, who has repeatedly been attacking Lalu and his family for purportedly owning undeclared properties worth over Rs 1,000 crore, alleged that the land dealings needed to be probed by Income Tax authorities as the documents in question were in the public domain.
Talking to reporters on the sidelines of his weekly Janata Darbar, the former Bihar deputy chief minister said he did not know about the current status of the property, which was valued at Rs 20,000 when it was allegedly gifted to Tej Pratap.
Asked whether he had asked his party MP Rama Devi about the issue, Mr Modi said the land deal took place long before she joined the BJP.
To a question, he said Bihar Chief Minister Nitish Kumar should reconsider his ties with the RJD and Congress.
Mr Modi said he would also complain with the EC against Lalu's other son, Deputy Chief Minister Tejaswhi Prasad Yadav, as he had also concealed details of his immovable assets in his election affidavit.
Reacting to the allegations, RJD spokesman Shakti Singh Yadav said Mr Modi had been confined to the footnotes of the BJP, and that he was making baseless charges against Lalu Prasad and his family in order to remain in the media glare.
"The allegations have no basis. Everything is in the public domain. If the donor has no problem in offering a gift, why is Sushil Modi hassled," he asked.