It should be IAS 36 Impairment of Assets.
This is because debtors are financial assets as per IAS 39 and debtor's write off is essentially an impairment.
Reversing an impairment loss for an individual asset is discussed under IAS 36.119 which says
"A reversal of an impairment loss for an asset other than goodwill shall be recognised immediately in profit or loss, unless the asset is carried at revalued amount in accordance with another IFRS (for example, the
revaluation model in IAS 16). Any reversal of an impairment loss of a revalued asset shall be treated as a revaluation increase in accordance with that other IFRS."
Hope your question is answered