I'm being required to receive approval from my manager to apply to a position with a partner company. Advice?

I work in inside sales & project management for a wholesale supplier for commercial contractors. Recently, I heard that a partner company that we operate as a rep for had their Territory Manager for our region quit. I've been working with my current company for about 2-1/2 years, and have a lot of background during that time with selling the products made by the partner company. Much of the time I've worked for my current company I have felt overworked, underpaid, and under appreciated. On the online application for the new company I met all of the basic qualifications and am coming into this with existing product & industry knowledge, albeit 2-1/2 years worth. I am 25 years old and have an undergrad degree in business. I applied online on Monday and heard back from the company's regional manager Thursday that in order for us to proceed with discussing the specifics of position, filing my application, and scheduling any potential interviews that I will need to be completely transparent and notify my manager. I told him that I understand since our companies have a partnership and I will be working with some people at my current company if I land this new job, and his concern is that if I leave on a bad note then bridges might be burned for their partnership. Of course he insisted that nothing is guaranteed once I notify my manager and begin interviewing, but my gut is telling me that this is a risk I am willing to take.

Also worth noting; my company relocated me to a different office about 2 month ago to fill in for an outside salesman who recently retired. I've only been working under this new manager for these 2 months, and I do not have a very close relationship with him as of yet. He also gave me a $2/hr raise a few weeks ago (the only notable raise I've gotten since starting working for this company). I've heard from current coworkers that this is a great opportunity to potentially move into this outside sales role, but my manager hasn't once mentioned a plan to move me into that outside sales position permanently, although it can be interpreted that way. Since this isn't even a guarantee that I will be given this promotion I feel like I have to look out for myself in this situation and explore opportunities that are more beneficial to me. This opportunity with the new company will give me more autonomy, the opportunity to travel and visit customers, and will allow me to work remote, which will eliminate my current 1-1/2 hour round trip commute I do daily.

Has anybody experienced something similar or have any concerns or words of advice?

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