There is this case which is similar to my problem, though maybe more definitive.
To give a bit of background, I am originally a developper, worked in Europe and loved my job in Automotive, now I am living in North America and got hired as system tester (so other side of the V-cycle) and not really happy with this role.
I was hired as permanent recently in my company after a little more than a year as an external consultant, during which several projects were refused to me, as I was consultant and so a risk. They really wanted me to join them a few months ago, but one thing I expressed was that I desired to come back to my original career path, as a developper in R&D. They agreed to this (nothing written) and said that for now my work as tester (more like QA engineer) was great and there were possibilities to change path later.
Thing is, since I started to work here, it got worse and worse, at first I was designing some tests and procedure for the system, now I am mostly writing documents, and again, they told me my job is great because my background allow me to design good procedures/details to test etc.
I don't want to stay in this QA position, after being hired I hoped it would give me more leverage to request a change, but they are thinking of some nice projects for people outside the team/company. Staying in my position is bad in my opinion as I will lose all my skills gradually and be unable to come back as a developper later.
I got recently a nice interview with a company that is looking for someone like me, to do almost exactly the work I was doing in Europe, so even if I didn't get the offer yet, I'm preparing to this possibility. Still, I like a lot my current company, I made friends there, they have nice products, they just see myself as efficient in my role and my team lead don't want me to leave to another department, which I understand.
How could I use this potential offer to push myself to a more appropriate role as engineer ? I don't want to blackmail them, I've read on TheWorkPlace that it was always best to negociate, and then tell them about the offer, not the other way.
Thanks a lot for your suggestions
EDIT : Just to be clear, I don't intend at all to pressure them, or blackmail, I meant that I want to find a win-win solution without getting to such extremes, as I said several coworkers are friends.
I see the other potential offer as a plan B if discussion gets bad ("We hired you for this task, just do it, or leave") or if they ask me why they should switch me to another position, are my skills good enough ?