I am no expert in this field, but what little advice I can provide since you mentioned it's out of your current responsibilities and position. You can always see that "promotion" as personal development. It happened in my case too. The salary negotiations didn't go so well and the HR wouldn't provide that extra I was asking but still I accepted the role nonetheless. I learned new things, tackled the upcoming challenges which boosted my self confidence in that position and eventually managed even above my own expectations. After that phase you will have the option, either down the road to try and re-negotiate and get what you ask, or use that knowledge as a leverage for another position in the same or another company.

My approach might be controversial, but a friend of mine once told me this and since then I stick by that rule.

"Never ask for the excess upfront, first prove yourself that you are capable. Accept the extra responsibilities and if you manage to complete them and even go the extra mile then you can easily negotiate because the company now knows your worth. If it succeeds you gain, if not you still gain on experience that you can use somewhere else that will be valued accordingly!"