"Decrease in Interest" should manually add Interest amount in "Gross Interest" section as tax facts to make the difference amount to zero amount. If there are differences between accounting and tax facts, the differences will be reflected in the Reconciliation section of the form. Increase/decrease in Interest should be nil.
This also applies to income received from Dividends, Trusts and Partnerships.
Guides:
Reporting Interest Income received into a Company Tax Return (CTR)