WWYD? (nwr)

So as I told you last week, I was offered a promotion at work. It's not so much a promotion as it is a very similar job but in a different part of the agency and it comes with a slight pay increase. I accepted, but I won't be able to start until at least the last week in October as my current boss says he can't let anyone go yet

BUT.......yesterday I got a phone call to interview for another position I sent an application to at another agency. It's a supervisor position (as opposed to my current case worker and soon to be case manager position), so it obviously means much more responsibility, but also of course, a higher salary. Essentially, it's my current boss' job, just at a new (new to me, but very stable and established) agency.

DH says I shouldn't even go interview because if I get offered the job, how am I then supposed to go back to my current agency and tell them (a) I'm leaving my current job (which they know) AND (b) I now declining the new position as well (after already accepting).

I think I need to go anyway and give it a shot. It's so hard to get supervisory experience in the social work field if you don't already have it, and it doesn't seem the managers at my agency are going anywhere anytime soon to allow for that growth option.

So ladies..........WWYD??????