Travel after H1B approval before start date

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Hi,

I'm currently on OPT and my future employer has filed an H1B CAP Exempt application for me with a start date of 8/1. The application was recently approved. But I need to travel to Canada for a conference in May, before the effective start date. My future employer advised me not to travel until 8/1, saying it will cancel the H1B approval. Is that the case? Is there any possible workaround? (I really want to attend the conference).

Thanks in advance!