Exchanges can't be canceled. Each member receives slots proportionally with the number of sent lots. When you received the address of a member, that slot is consumed. If you don't sent the stamps to that member, that member will actually be losing one slot.
It's important to send the stamps even if they don't meet entirely the preferences of your partner. But keep in mind the quality rules.
Also, if a member prefers some themes or years that are harder to find, that member understands this and probably already expects not to receive too many of the specified stamps.
If you try as much as possible, it should be ok.