i was just wondering to trade with someone do you have to have the exact amount like if i put up 20 blue d hides and someone wanted 100 blue d hides would we exchange?

yes

so he would end up with 20 d hide and some of his money back?

 

 

 

maybe there should be an option on whether to accpet different amounts?

Jagex fills orders from multiple users. So in your case they would take your 20 and 80 more from another player(s).

addition to person above me: they also take cheapest offers so if you offer 200 gp per hide and someone offer hides for 180 ea you get 20x gp back (with x being amount of hide (200 hides = 20*200=4000 gp back))