I am Korean. I think that .. hum ...
It might be meaning of ‘ I want to remain good friend and good friendship with you’.
That dosen’t mean situation in store, buying something.
앞서거니 뒤서거니 계산하지 않는 사람 means ‘who is not calculative with one thing or another thing whatever’.
Whether it's good or bad situation, I'll be your friend. In other words, I'll be with you , no matter what the situation is.
Korean people can understand the meaning of 앞서거니 뒤서거니 , but it is not common expression to be used.
It is kinda little difficult sentence. I don’t use this sentence often.