It depends on the broker, and I am not sure about IB. Some brokers simply
do not allow you to use the unsettled money. Some brokers will let you buy,
but you cannot sell the newly-bought shares before the old shares are
settle; otherwise you will have a free rider.
The solution is to have a margin account. As long as you are not using
margin, there is no interest. Even you use margin, there is still no
interest if you close your positions, bought with margin, at the end of the
same day. However, sometimes it is difficult to resist the temptation of
using margin once you are allowed to use it.