TL;DR version: Company is in a bind and needs an assistant manager/manager ASAP. I can do the job with no training needed starting next week and going until the fall when I start school again. How do I leveraged the situation in my email offer to take the job to get the best deal?
The long story:
I've been with this company for 15 years, including as an assistant manager for 1.5 years. I recently took a part-time role at a new store so that I could take a part-time teaching position. I've been at the new store since September and it has been a RIDE. The manager who hired me here got transferred to another city the week I started, and we've had a revolving door of new managers in the months since, of varying quality/competence (I think we're up to three now?). I've kept my head down during this, and focused on my section of the store except for critical circumstances. I've consciously avoided doing management tasks, though I have agreed as a resource for the new managers on institutional knowledge.
Now the store's ASM (who's been passed over for promotion at every step) had been named Acting Manager, and the company is apparently haven't trouble hiring someone to fill the management vacancy. I can step into an Acting ASM role basically immediately, as school is finished in two weeks. I've yet to talk to anyone but the acting manager about taking the role, and she's supportive of me going for the best pay I can. How do I frame all this in an email to our district manager for the best chance of success?