I am a US non resident importer who would like to donate some of my products to one of my clients in Canada to build a relationship. Do I have to pay duty?

Yes, you would pay the duty. Here is the information regarding donations going to a charity which include, donations of clothing and books for charitable purposes; Donations of any goods by non-residents of Canada to religious, charitable or educational institutions in Canada; Photographs, not exceeding three, sent by friends and not for the purpose of sale.