What does give exchange mean?

I need help an iPhone give exchange is 333 and cex offered 90 for my blackberry if I give the bb do I pay 333 on top or take away the 90 from the 333?

I would expect they would take the £90 off the £333, with it being an "exchange". You're probably best asking them what the terms of their exchange are. I can't see them taking your phone and expecting the full £333 for the iPhone.