Canceling - 12 days before the trip

I met the woman last week - was hesitant - because I wouldn't be able to work in person and the small dog needed to be watched very closely.

I thought it could be worth it - if I stayed at the house and studied the 5 days.

Fast forward to yesterday - I got a PT job that has a weekly schedule.
Accepted verbally.
The dogsit would be the 2nd week of my job.

How would I let my manager know?
I probably would have to cancel the dog sit? 09/12 - 09/17

Also -- is that I would make slightly more via the dogsit-- but having my manager respect me is more important.
I just feel like it might be too late to cancel.
but maybe someone will be able to pick up the gig.