As a missionary, you are considered doing clergy work and it is taxable, should be reported to you in box 7, Form 1099-MISC as Nonemployee Compensation. You should record it as such even if the amount is shown in box 3 in error. The money is raised to enable you to do the work and is required to be reported for federal tax and for self employment tax (social security and medicare taxes). Although you consider it a "volunteer activity", you are being paid and using the money to live on, which indicates you fall under the "missionary" reporting rules.
The detailed discussion by both of the posts below will help you better understand the process for clergy work and the tax reporting. This will provide insight into the twists and turns of the tax rules for missionaries.
"Even though you are not affiliated with a specific church and are not recognized as employees or contractors of a specific church, being a "missionary" you are considered as clergy by the IRS. This would mean that the earnings reported in Box 3 may be reduced by an agreed Housing Allowance or Parsonage if the churches with which you engage have previously set to record an amount as Housing Allowance. As for liability for Self-Employment Tax, that is Social Security and Medicare Taxes, you are responsible to pay these taxes, at the total rate of 15.3% on the amounts in Box 3. Please read the following:
"A licensed, commissioned, or ordained minister is generally the common law employee of the church, denomination, sect, or organization that employs him or her to provide ministerial services. However, there are some exceptions, such as traveling evangelists who are independent contractors (self-employed) under the common law. If you are a minister performing ministerial services, all of your earnings, including wages, offerings, and fees you receive for performing marriages, baptisms, funerals, etc., are subject to income tax, whether you earn the amount as an employee or self-employed person. However, how you treat expenses related to those earnings differs if you earn the income as an employee or as a self-employed person."
"For social security and Medicare tax purposes, regardless of your status under the common law, the services you perform in the exercise of your ministry are considered self-employment earnings and are generally subject to self-employment tax."
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