Can he get rid of interest by giving it to his married daughters?
Putting money in the bank in return for interest is emphatically haraam and is a major sin. Allaah says (interpretation of the meaning):
“O you who believe! Fear Allaah and give up what remains (due to you) from Ribaa (from now onward) if you are (really) believers.
279. And if you do not do it, then take a notice of war from Allaah and His Messenger but if you repent, you shall have your capital sums. Deal not unjustly (by asking more than your capital sums), and you shall not be dealt with unjustly (by receiving less than your capital sums)”
It is not permissible to put money in a riba-based bank except when there is the fear that the money may be stolen etc., if there is no means of protecting it other than by putting it in the bank. In that case it is sufficient to put it in a current account that bears no interest, because cases of necessity may make forbidden things permissible, but only to the extent that is necessary.
The one who has fallen into riba must repent to Allaah, may He be exalted, by giving up the sin, regretting what he has done, resolving not to do it again, and getting rid of the haraam interest by donating it to charitable causes. He does not have the right to benefit from it for himself or for those on whom he is obliged to spend.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) said: As for the interest that the bank gives to you, do not return it to the bank and do not consume it; rather give it to charitable causes such as giving it to the poor, repairing public washrooms, and helping debtors who are unable to pay off their debts. End quote from Fataawa Islamiyyah (2/407).
There is nothing wrong with giving this money to your married daughters, if they are in need of money, because you are not obliged to spend on them, rather their husbands are obliged to do that.
And Allaah knows best.