I would just walk away.
I had a phone connect (so diff situation) and my friend would drive out to me and we would meet my connect. My friend was stubborn, I give him credit. If our connect wasn't there, he would straight up leave and head home. I would call back the connect, and tell him to cut the order in half b/c my friend left, and he would always tell me to call him back and tell him he would do it for cheaper (not getting into pricing, but he really did lower the price).
I know it's different cold copping, but walking away proves that
a) your not a cop, as I think you posted b4 that some corner boys tried calling you out, and
b) you aren't a desperate junkie, so to them you are probably better business than the fiend that is begging for lower prices and to be spotted a few bags. Instead you just want your moneys worth, and have the time and patience to get just that.
Also, I would use the "I only have $X on me, can you do the full bundle (I think u guys call it a jab) insted of me having to come back later". If what you have on you is close, then they should do it to avoid more heat w/ you coming back, and also since by me anyway they don't wanna have to break up bundles and remember how many stamps (in my case) are banded together.