Donation deduction under Section 80G remains protected for donors unless exemption certificate is withdrawn promptly. Explanation 2 protects donor entitlement to deduction where, at the time of donation, no part of the institution's income was chargeable to tax; donors remain entitled despite subsequent changes unless the exemption certificate is withdrawn. The I.T.O. must immediately inform the Commissioner when a trust will not satisfy conditions for tax-exemption so the certificate can be withdrawn and donations thereafter will not qualify for deduction.