I'd recommend retaining a lawyer as soon as a large company like Microsoft reaches out to you. (Even if it's just for your private education of risks, rights and strategy.)
Alternatively, start billing for time and travel immediately.
If they don't want to pay, tell them to meet you locally. (Let them incur travel expenses.)
This will force the company to demonstrate their intent.
If they balk at both requests, this shows that:
1. They don't have managerial approval for their escapade.
Or...
2. They intend to take advantage of you to the extent that they can.
Either way, it shows they need to get their ducks in a row. And prepare themselves to treat your time as valuable.
Alternatively, start billing for time and travel immediately.
If they don't want to pay, tell them to meet you locally. (Let them incur travel expenses.)
This will force the company to demonstrate their intent.
If they balk at both requests, this shows that:
1. They don't have managerial approval for their escapade.
Or...
2. They intend to take advantage of you to the extent that they can.
Either way, it shows they need to get their ducks in a row. And prepare themselves to treat your time as valuable.