This is the verb to barter with the addition of three points of light. A point of light signifies that something is true. Therefore, each side of the deal agrees to what is being exchanged in the centre. Thus, Mahadevan's sign number 406 is an ideograph for an agreed deal.
Illustrative Text Reference:
Lothal: Seal: L-110 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 265: Collections in India: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 406: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.