I'm currently a postdoc, and someone from another institution (a colleague of a colleague; I didn't know them before) contacted me to invite me to collaborate on their project.
While the project seems interesting, I would need to invest my own time outside my normal working hours, so I'm not willing to take it without proper compensation.
Basically, I would like to ask for a fee. The person who contacted me said something a bit ambiguous "we have no money, but we could probably get some". Do you have any advice on how to politely ask for a fee?