"If he knows this, he will get a prize", "If he loves me, he will propose", "If you listen to this music, you will like that"
All these sentences are fine and idiomatic.
As I read the rule about first conditional, "if" always implies a future event (future - future)
The rule is an oversimplification. I was unfamiliar with the term "first condition" and so googled it and found the terms "zero, first, and second conditional." This framework describes the most frequent usages of conditional clauses but leaves many less frequent types out.
The "if" clause is not limited in use to future reference but can be used for present or even past reference. For example,
If it is raining now, we will have to cancel the picnic later today.
The grounds will be too muddy. (This sentence presumes the speaker cannot see whether or not it is raining at the moment of speaking.)
If it rained last night, we will have to cancel the picnic later today. The
grass will be too wet. (This sentence presumes the speaker has not yet seen evidence of last night's weather.)
These sentences contrast with future reference, such as:
If it rains tomorrow, we will have to end the picnic early.
The above uses present events as possibilities that should be considered within the context of the discourse. If you want to describe possible events that are only hypothetical or even contrary to fact, you would say:
If it were/was raining now, we would have to cancel the picnic later
today. The grounds would be too muddy. (This describes a situation contrary to the actual facts.)
If it had rained last night, we would have to cancel picnic later today.
The grass would be too wet. (This sentence presumes the speaker has not yet seen evidence of last night's weather.)
If it rained tomorrow, we would have to end the picnic early.