Money can subvert anything, including missions. If you are being paid, your work will be motivated at least in part by your need for income and your need to please your employer or sponsor, sometimes at the expense of what you believe is best for the mission.
If you are paid and your work is at all political, you could be accused of taking a political position for financial gain. This could undermine your credibility in the eyes of the people you are trying to serve, the public or potential allies, especially if you are a foreigner.
If you do not already have the funds for your mission, you may need to spend a considerable portion of your time either trying to find paid work or raising funds, instead of working on the mission’s projects.
Also, any payment you receive for mission work could take away scarcely-available funds from capable local people who may have fewer options and need the income more than you do. So while trying to help, it is best to avoid this kind of harm.
Missions require funding, so if you are not being paid then you will need funds from another source. In my case, this has been from my retirement accounts. Other sources for self-funded missions could include proceeds from a lucrative career, a windfall from investments or an inheritance.
The amount of funding required for mission work can vary greatly depending on the type of work you do and your location. Given the available funds, certain kinds of work and specific locations may be feasible and others not.