Is it obligatory for someone who has dealt with an Islamic bank that uses treasury bills to purify their earnings?

If the bara' (clearance/disengagement) you are dealing with observes the Shari'ah (Islamic law) regulations in its transactions, then the default ruling is that the profits resulting from the investment are permissible. However, if you come to know that a portion of the profits originated from prohibited transactions, such as treasury bills, then you must dispose of the prohibited percentage by giving it to the poor and needy. It is advisable to consult the bank to ascertain the nature of the transactions and the percentage of prohibited profits.