Yasir told his wife, 'if you repeated those particular words (something he dislikes), that will be the last day of their marriage'. He is asking whether he can revoke his words. His wife has changed her ways since then.
In the name of Allah, the most Beneficent, the most Merciful.
If one connected the divorce to the occurring of a condition, divorce will occur when the condition takes place. And the oath (i.e. the connecting of the divorce) will come to an end, because when the condition takes place, the connection of the divorce no longer remains. (Hidayah p.385 v.2)
Regarding your question, if you did not use the word whenever, then divorce will only come into effect when that thing happens the first time, after which the oath comes to an end and no further divorce takes place again.
However, if you said: whenever you do this, you are divorced then whenever that thing happens, one divorce comes into effect. Thereafter, if you take her back and she does that thing again, once again a divorce comes into effect until it reaches three divorces. (Fatawa Hindiyyah p.415 v.1)
In a nutshell, a conditional divorce cannot be revoked. The only time it comes to an end is when the event happens. It should be borne in mind that the aforementioned ruling will only apply if the husband took the meaning of the word “last day of their marriage” to be in the meaning of divorce.
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham