Is repentance valid if it is contingent upon the occurrence of a sin, such as repenting from lying about prophets before it happens, provided there is regret and a firm resolve not to repeat it?

Repentance occurs after committing a sin, not before it. What precedes the sin is an intention to repent, not repentance itself. A Muslim should intend to obey Allah, perform the enjoined acts, and abandon the prohibited ones, and should hasten to repent whenever they sin. If one sins and does not repent, they are not considered repentant merely by their prior intention; rather, repentance must occur after the sin, as Allah Almighty says: "But only repentance is due to Allah from those who do evil in ignorance and then repent soon after."