I spoke to Alliant CSR and was told that there will be no retrospective interest charges.
Also, they charge interest on a monthly basis and as long as there is no balance when the next statement cuts I will not be paying any interest.
First part is also confirmed by MagnifyMoney:
What happens if I don’t pay off the balance transfer at the end of the promo period?
Once the introductory period is over, interest will start to accrue at the standard purchase interest rate on a go-forward basis. Interest during the introductory period is waived — so you do not need to worry about a retroactive interest charge.