동사 + 은/는 김에
In the following sentence, I don't understand why the 김에 tense that is used is used.

'우리 만난 김에 식사나 하자.'

My confusion is, if 하자 is a suggestion, as in an action for the future, and 은 김에 is to be used with past tense verbs, why are the used together here.

Shouldn't the 김에 form be future tense?

Question

동사 + 은/는 김에
In the following sentence, I don't understand why the 김에 tense that is used is used.

'우리 만난 김에 식사나 하자.'

My confusion is, if 하자 is a suggestion, as in an action for the future, and 은 김에 is to be used with past tense verbs, why are the used together here.

Shouldn't the 김에 form be future tense?

Answer

The act of meeting has already happened so it should be expressed in past tense. 
You use -(으)ㄴ 김에 in past tense sentences and -는 김에 in present and future tense sentences.

You can use -(으)ㄴ/는 김에 when talking about something (usually not originally planned) that you do while you are doing something else, because it’s convenient, effective, or easy that way.

'우리 만난 김에 식사나 하자' makes sense because the speaker wants to eat together as they met even if he/she didn't plan it before they met.

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