Shaikh Ibn Bāz: It is not permissible for a Muslim, while he is in a state of ihrām, to apply perfume, neither to his head nor to his body. He must refrain from this, because the state of ihrām prohibits perfume until he exits his state of ihrām: until he completes his release from his ʿUmrah, or until he completes his release from his Hajj by performing two of the three acts of release.
So, applying perfume while in a state of ihrām is not permissible. If he does so deliberately, then he must offer the fidyah (expiation), which is either:
1. Feed six poor people,
2. Slaughter a sheep,
3. Or fast for three days.
However, if he was unaware of the ruling or had forgotten, then there is nothing upon him.
The point is that he may not apply perfume after entering ihrām. Rather, perfume is used at the time of entering ihrām, before actually entering it; it is recommended for him to apply perfume when preparing for ihrām. Then, after completing his release from ihrām, he may apply perfume again.
But after entering ihrām, he may not apply perfume. Once he has formally entered ihrām—that is, once he has intended to enter into the rites of Hajj or ʿUmrah; this constitutes entering ihrām. In other words, after intending to enter the Hajj or ʿUmrah (at its proper place), he should not apply perfume thereafter. Yes.
Ibn Bāz also explicitly states elsewhere that if a person performs a prohibited act while ignorant or forgetful, such as applying perfume, there is no fidyah upon him.
Shaikh Ibn Bāz (rahimahullāh) also said about the compensation: “Either fasting for three days anywhere; or feeding six poor people, giving each poor person half a Sāʿ (1.2 kg); or slaughtering a sheep in the Haram in Makkah.” The meat is distributed among the poor of the Haram. And you may appoint a trustworthy person to undertake this on your behalf.
Source: From Shaikh Ibn Bāz’s official website.