Is the amount that falls below the nisab due to temporary lending, then is returned on the same day or the following day, considered to have had its hawl (zakat year) interrupted, and thus a new hawl begins?

If money reaches the zakat threshold (nisab), lending it does not interrupt its hawl (one-year period). It makes no difference whether the borrower repays the money or not, or whether they used it to fulfill a need or not. Zakat is obligatory on the lent money because the lender's ownership of it has not ceased; its equivalent remains as a debt owed by the borrower. Therefore, the questioner must base their calculation on the money's hawl and pay zakat on it when its hawl is completed from the date it reached the nisab.