trade lasts

a compliment that I heard about you that I offer to trade for a compliment you have heard about me; can be redeemed at a later date or used as an installment for the future (as in, when someone hears something nice about you, if you have given them a trade-last, they are bound to tell you the compliment you received but weren't around to receive it)
> Me: "Hey! I heard Peggy thought your butt looked great in those jeans."
> You: "Wow, really? That's awesome!"
> Me: "Yeah, that's a trade last, so now you have to tell me next time someone says something good about me."
> You: You got it!
> (six months later)
> You: "Hey! I heard Peggy thought your butt looked great in those jeans."
> Me: "Wow, really?"
> You: "Yeah! That Peggy sure is obsessed with butts."
> Me: "She's not very discerning, either."